ARLINGTON, Texas -- This one will linger for a while, hanging over this USC football team throughout the offseason, spring, fall camp and really up until the next time the Trojans are challenged in a big matchup.

At least the questions and concerns about the defense will ...

When No. 10-ranked USC went up 15 points with 4:30 left in the fourth quarter of its Cotton Bowl showdown with No. 16 Tulane on Monday, ESPN analytics gave the Trojans a 99.8-percent win probability.

From there, literally everything went wrong for the Trojans.

That's the only way this outcome could have happened, that the Green Wave could erase that deficit and take its first and only lead of the game with 9 seconds remaining, delivering a stunning 46-45 knockout to USC.

"As tough a loss as I can remember in my entire career," Trojans coach Lincoln Riley said.

"It's going to linger," quarterback Caleb Williams acknowledged, the frustration visibly pulsing through him. "... You lose the last game of the season, go into offseason, burns. Don't have another game after and, yep, it's going to burn."

"Definitely after the game, like man, how did we let that one slip away?" edge rusher Nick Figueroa said "We didn't get the stops we needed in the second half, we didn't play very well. ... I wouldn't say there was evaluation in the moment, but after the fact it's like, 'Man, we're one stop away, you've just got to make a stop on all those opportunities.'"

Or any of them.

RELATED: Everything Lincoln Riley said after USC's Cotton Bowl loss to Tulane | WATCH: Postgame interviews with USC players

Again, literally everything had to go wrong in those final four and a half minutes to get this outcome.

It started after a Denis Lynch field goal put USC up 45-30, as Tulane immediately responded with a 23-second scoring drive, on a 59-yard pass from Michael Pratt to Duece Watts and a 4-yard Tyjae Spears touchdown run to make it a 45-37 game.

Mario Williams, who had only one official kickoff return all season, then dropped the ensuing kickoff leading to the Trojans having to start at their own 1. Austin Jones rushed for 0 yards on his first attempt and then got stuffed in the end zone for a safety as Caleb Williams tried to take the ball back from him to salvage the play.

"It's sitting heavy in my gut right now. Our unit didn't get it done when we needed it most, and [we] take responsibility for that," center Justin Dedich said.

Said Caleb Williams: "Tried to make a play happen. They were backed up, Austin got stopped, I tried to get it out of his hands somehow or something and it didn't happen -- safety."

Now Tulane trailed 45-39 with 3:20 left to play and was getting the ball back with a chance to suddenly steal the game after trailing the whole day.

Immediately, Tuli Tuipulotu sacked Pratt to set up a second-and-21 that would then become a fourth-and-6 -- one of those opportunities Figueroa referenced -- but Pratt scrambled for a gain of 8 and the first down before linebacker Raesjon Davis, who had dropped partially into coverage, could get to him.

On the next play, Figueroa sacked Pratt and soon again it was fourth-and-10. Another opportunity ... but ultimately another first down as Pratt found tight end Alex Bauman for 24 yards down to the Trojans' 30-yard line.

Pratt hit Watts for 24 yards down to the 6, and on a play that started with 12 seconds on the clock Bauman then dropped a short pass. He wasn't trying to, he said afterward, but it was fortuitous because if he caught it he almost surely would have been tackled in bounds with Tulane out of timeouts. But he didn't ...

Instead, he caught the next one -- a 6-yard pass into the end zone with linebacker Eric Gentry draped around him. It was initially ruled incomplete, but Gentry's arm was under Bauman and actually helped keep the ball from hitting the turf before the tight end could fully secure it against his stomach. The replay review changed it to a touchdown, and the extra point put the Green Wave in the lead for the first time all game -- the only time that ultimately mattered.

"I've rarely at the end of a year felt so conflicted," Riley said. "On one hand, I'm sick about the way that we finished the season, particularly the fourth quarter of the last two ballgames. We lost three games this year -- we lost two of them on the last play of the game and we lost one in the fourth quarter of the championship game where we had a chance to go to the College Football Playoff. The competitor in you, the competitor in us, that's a tough pill to swallow, man, because those opportunities are not easy to get.

"Now, the flip side of it is we were in those positions, and those positions are very, very real. We're in this locker room, we're here at the Cotton Bowl in a New Years Six game, and again 12 months ago I don't think many people on the outside believed that was even possible. So I think getting through this first season, there's a ton of great momentum in this program, and today's result -- as sick as it is -- does not change that. The reality is that we're here and we're not going anywhere. We know what we need to get better at as a program. We have to finish better, and we didn't get that done enough this year."

It is indeed conflicting because Riley and these Trojans (11-3) did pull off a remarkable seven-win improvement from the 4-8 team he inherited. They did get to the brink of a potential College Football Playoff berth before coming undone against Utah in the Pac-12 championship game, with their Heisman Trophy-winning QB playing through by a significant hamstring injury most of that game.

Back closer to full strength Monday, Williams did everything he could to not only dominate Tulane's defense but also try to overcome his own.

He completed 37 of 52 passes for 462 yards and a Cotton Bowl-record 5 touchdowns, with 1 interception. He led the Trojans to 7 touchdowns, a field goal and a missed field goal within their first 10 possessions. USC was 11 of 15 on third-down conversions.

But the Trojans defense was that bad Monday that it wasn't enough. Especially as the game wore on ...

Tulane (12-2) didn't punt after halftime, scoring 4 touchdowns, a field goal and losing a fumble on its final six drives. The Green Wave ran 32 fewer offensive plays than USC but still put up 539 yards, averaging 10.4 yards per play overall and 9.0 yards per rush. Tulane only completed 8 passes, but they went for 234 yards.

They had scoring drives of 2 plays for 92 yards (with an 87-yard touchdown to Jha'Quan Jackson for the longest pass play in Cotton Bowl history), 3 plays/65 yards and 2 plays/63 yards.

Williams was asked about the toll that took on the offense ...

"I don't know. Our job on offense is to score. That's all I can control, we can control. You can lead from the sideline and lead when you're on the field, other than that can't control that," he said.

(He later made a point to credit the play of the defense this season while noting that unit took the lead at times along the way when the offense was lagging.)

Riley was asked afterward why he thinks the defense fell apart these last two games --- really three of the last four -- allowing at least 45 points and 500 yards in each, and where his confidence stood in defensive coordinator Alex Grinch moving forward.

"We just got done playing a game 15 minutes ago so I'm not going give any big picture assessments. We didn't play good enough today," Riley said. "... The big plays are obviously, and the lack of, just not tackling to the level we would expect, are very, very alarming and something we've got to do a much better job of. ... We'll take a deep dive into every part of it here over the next couple of weeks. We'll lay out a plan, just like we did when we landed in LA 13 months ago -- you take a deep dive of it, you assess what you got and how can we make it better and then you go to work.

"Now we just repeat the process knowing a little bit more about this roster and a little bit more about where we're at and what we have coming back and what we're going to add. You know, we made a pretty good jump in Year 1, and I wouldn't bet against us to make another big jump next year."

Given Riley's track record -- he's now 66-13 in six seasons as a head coach -- it would indeed seem unwise to doubt the overall trajectory of the program.

But the defense is a disaster, having now given up the most yards per game in program history (423.9), which is significantly up from the 408.9 last year in that hapless, lost season.

Earlier in the season when the unit was offsetting giving up big plays by making even bigger turnovers, it was a different story. The defense was even directly responsible for two of the Trojans wins (at Oregon State and vs. Notre Dame).

But down the stretch it all came undone and now only a mess of questions remain.

"I don't know that anything's changed. We haven't changed anything dramatically. I would say maybe just us not trusting our foundation as much as we should, whether that be the Trojan standard or be it the defense, its standard. We've just got to try to rely on each other more, I guess, delegate more than everybody trying to be Superman, I guess," linebacker Shane Lee said.

What has caused that lack of collective trust?

"I'm not sure. Initially, off the top of my head, I can't really tell you," he said. "It's probably different for everybody, honestly. I got to look at myself and see what is for me. That's something for us to focus on moving forward this offseason."

Figueroa couldn't put a finger on it either, why this defense lost its way so jarringly down the stretch.

"I'm not sure. I think sometimes guys just lost the edge of 'I've got to be the one to make the play every time the ball is snapped.' And that's just something that's got to be thought of more by the guys every time they line up -- like, this is an opportunity for me to make the play," he said. "Yeah, we've got to be better."

Riley, meanwhile, emphasized the big-picture perspective, as is also warranted.

USC took major leaps from the 4-8 nadir the program hit last season prior to his arrival. That shouldn't be forgotten. He and this team accomplished a lot. The last two games don't change that -- they just make it abundantly clear that as close as the Trojans may have been to a playoff berth this year, they still have quite a ways to go to truly be a playoff-caliber team.

Riley's confidence wasn't compromised after the loss Monday, as ugly as it was.

"We know what we're doing. We know the areas we have to get better. We know the things that we have to address. We'll spend a lot of time on it, get some clarity, let this one kind of soak in and then get past it and we'll go back to work," he said.

"We didn't come here for a one-year deal. We knew that it wasn't when we signed up to come to USC. This one hurts, but also it gives complete clarity on what we need to do next, which is go back to work and keep building and developing our program."

1st Quarter

USC, 5:58: Michael Jackson III 9-yard pass from Caleb Williams (Denis Lynch kick). 7-0, USC

2nd Quarter

USC, 12:33: Terrell Bynum 3-yard pass from Caleb Williams (Denis Lynch kick). 14-0, USC

Tulane, 9:30: Tyjae Spears 3-yard run (Valentino Ambrosio kick). 14-7, USC

Tulane, 5:42: Jha'Quan Jackson 87-yard pass from Michael Pratt (Valentino Ambrosio kick). 14-14

USC, 2:21: Raleek Brown 39-yard run (Denis Lynch kick). 21-14 USC

USC, 0:12: Brenden Rice 4-yard pass from Caleb Williams (Denis Lynch kick). 28-14 USC

3rd Quarter

Tulane, 11:55: Tyjae Spears 7-yard run (Valentino Ambrosio kick). 28-21 USC

Tulane, 4:02: Valentino Ambrosio 42-yard FG. 28-24 USC

USC, 1:24: Brenden Rice 19-yard pass from Caleb Williams (Denis Lynch kick). 35-24 USC

Tulane, 0:40: Tyjae Spears 3-yard run (2-pt failed). 35-30, USC

4th Quarter

USC, 12:09: Kyron Hudson 4-yard pass from Caleb Williams (Denis Lynch kick). 42-30 USC

USC, 4:30: Denis Lynch 43-yard FG. 45-30 USC

Tulane, 4:07: Tyjae Spears 4-yard run (Valentino Ambrosio kick). 45-37 USC

Tulane, 3:20: Safety. 45-39, USC

Tulane, 0:09: Alex Bauman 6-yard pass from Michael Pratt (Valentino Ambrosio kick). 46-45, Tulane

Up by 8 with just minutes remaining, the Trojans seemed poised to salt away the game as they set up to return a Tulane kickoff. Inexplicably, Mario Williams allowed the football to bounce straight off his chest and out of bounds at the 1-yard line.

With the Trojans forced to fight their way out from the shadow of their own goal line, a safety two plays later turned the game on its head, putting the Green Wave within striking distance down just 6 points and sending the ball back to Tulane with a chance to win the game.

With just seconds remaining, Tulane made good on that opportunity and completed the scoring drive, downing the Trojans in a collapse for the ages.

The dropped kickoff from Williams was just one in a whole slew of disasters for USC, but it was the pivotal and unexpected point upon which the direction of the game changed.

Wide receiver Brenden Rice

It took 14 games to get there, but Brenden Rice finally had the kind of monster game his potential has always suggested. The frustrating drops and failure to play up to his size were gone at last, as Rice used every bit of his strength, speed and leaping ability to put on a show, making one highlight reel play after another.

He made a sensational high-point catch to tear the ball away from a defender early in the send quarter for a 30-yard gain on third-and-22, setting the Trojans up at the goal line for their second score and giving himself a jolt of confidence he'd ride the rest of the game.

On USC’s final drive of the half, Rice made his presence felt again with another leaping grab on a back shoulder fade, this time following up with a violent cut to the sideline to leave his defender in the dust before taking the ball out of bounds for a gain of 35-yards. Rice capped off the drive on third-and-goal, racing down the back of the endzone to allow a waiting Caleb Williams to hit him for the teams’ fourth touchdown of the day.

He made another series of remarkable plays down the stretch, hauling in a bomb down the field for a 74-yard gain and making a sensational catch for a 19-yard touchdown in the second half as he stretched the ball over the goal line before going out of bounds.

Rice finished with 6 receptions for 174 yards and 2 touchdowns, easily the best showing of his career and a performance that he can hopefully build more confidence from going into this year.

Cornerback Mekhi Blackmon

There wasn’t much to be said for positive play on the defensive side of the ball for USC, but Mekhi Blackmon was rock solid all day long, manning his cornerback spot as well as anyone could have asked of him.

As a tackler, Blackmon was the only Trojan who had a day to be proud of, getting off blocks and making secure tackles on the perimeter with consistency. His instincts for getting to the football paired with the reliability of his tackling form allowed him to make stops when given the opportunity, helping to shut down a handful of runs to the outside and even blowing up a quarterback keeper off the edge.

The senior didn’t disappoint in coverage either, refusing to allow a single completion in his direction despite facing a handful of targets. Blackmon did an excellent job matching a receiver down the sideline on a first half pass attempt intended for the end zone, finding the football and breaking it up with one hand. Later on, he plastered his man on a goal line snap, smothering the out route to force an incompletion. He’d go on to make the biggest defensive play of the day for USC, stripping the ball free from Tyjae Spears in the fourth quarter (and recovering the ball) to stop yet another Tulane drive that appeared destined to end in a score.

Blackmon was the most reliable player on the Trojans defense from start to finish, making all the plays required of him and then some. Unfortunately, it wasn't contagious.

Brenden Rice’s second touchdown catch of the day was an absolute stunner from start to finish. Caleb Williams rolled left to escape pressure from the pocket, locating a crossing Rice as he approached the near sideline and firing the ball off platform 17-yards down the field and perfectly into his receiver’s outstretched hands. Rice’s hands locked onto the football while his lower half adjusted rapidly, his steps chopping upfield to stay inbounds as he extended his arm just over the pylon for a USC touchdown.

USC’s defense was absolutely atrocious in this contest. This was easily the group’s worst performance of the year; a statement that’s hard to believe given some of the duds they laid earlier in the season, but this was as bad as it gets. Whatever tackling woes were on display in the disastrous Pac-12 championship loss were exacerbated tenfold against Tulane’s offense, as the Trojans appeared utterly unable to bring down ball-carriers. There wasn’t any way to justify or make sense of it; USC’s defenders simply could not tackle, failing time after time to make stops even when in perfect position.

Though the offense was electric all day, the defense’s total inability to get a stop made it impossible for the team to build any kind of safe lead as Tulane continued to claw back into the game with score after score. In the final stretch of the contest, the Trojans defense made egregious errors in coverage as well, allowing Tulane to escape two fourth-and-long situations despite big moments from the pass rush.

It ultimately overshadowed what had been a spectacular performance by Caleb Williams and the offense. Whatever excuses there were to be made for the defense are totally gone now; this was an indefensible performance by every standard imaginable. Whatever struggles the Trojans have had there all year, they’d never been quite this bad. The rapid collapse in the final moments was fated -- you simply can’t expect to win with that kind of performance on the defensive side of the ball.

There was some consternation about whether or not the defense was salvagable heading into this game, and there were decent arguments on either side. Despite their woes against Utah and the general flaws evident throughout the year, stronger performances against UCLA and Notre Dame as well as some consistent positivity in turnover and sack creation made a case for an upward trajectory on that side of the ball going forward.

After this game, it's abundantly clear that things need to be completely rebuilt for the defense. The unit should be much better than what it was on the field today -- there isn't a single acceptable reason for it to have performed this way in a game of this magnitude.

The offense was every bit as productive as it could have been expected to be -- there were no surprises there. The fact that kind of performance could still result in a loss speaks volumes as to just how bad things got for the defense today.

This is not the kind of trajectory you can take into an offseason as a coaching staff heading into what's supposed to be a playoff-run worthy season next fall. Riley has some tough decisions ahead.

Passing

Caleb WIlliams: 37 of 52, 462 yards, 5 TD, 1 Int

Rushing

Raleek Brown: 6 carries, 61 yards, 1 TD

Austin Jones: 22 carries, 61 yards

Receiving

Brenden Rice: 6 rec, 174 yards, 2 TD

Tahj Washington: 5 rec, 109 yards

Michael Jackson III: 4 rec, 31 yards, 1 TD

Terrell Bynum: 3 rec, 26 yards, 1 TD

Kyron Hudson: 4 rec, 23 yards, 1 TD