Barely three days into fall camp and the No. 2 ranked USC Trojans are already facing a potential devastating setback.

During the team stretch portion of Friday's practice, junior quarterback Mark Sanchez was assisted off the field by two trainers and escorted to the triage tent.

A large bag of ice was placed on Sanchez's left knee before he was loaded onto a cart and taken to Heritage Hall for further treatment.

Sanchez emerged later from the USC locker room in a clean shirt, a pair of crutches and a full leg brace that immobilizes the knee.

"It was right in front of me. He does the same thing every day, he throws the ball to Clay (Matthews) when we warm up," quarterback Aaron Corp said. He just went down three yards in front of me and I just hopped over him and kept running.

"At first I thought he was messing around, but he said, 'I am not joking here.'"

Around 200 fans attended the open practice, watching the team work out from the sidelines. Even though practice continued on the field, all conversation revolved around Sanchez's injury.

"He came down on the side of his foot and kind of twisted his knee," Pete Carroll said after practice. "He has already gotten checked out and we are hoping to get some news late tonight."

Arkansas sophomore transfer Mitch Mustain, redshirt freshman Corp and walk-on Chris McCaffery handled the quarterback duties for the remainder of practice.

Former quarterback and current wide receiver Garrett Green was also seen warming up his arm during practice.

"What it means for us is it is an extrodaniary moment and an opportunity for these other young quarterbacks," Carroll said. "My thought is we are so lucky to have a guy like Mitch and a guy like Aaron that can jump in and take their shots at this thing until Mark is back in here and battling with them."

At least for the time being, the quarterback competition that was thought to be closed has now been blown wide open.

"I don't know how bad it is or how long we will have, but while we do have it, we have to take care of business," Mustain said.

Update: Pete Carroll returned to say that Mark Sanchez suffered a dislocated left kneecap that was put back into place with no complications. Rehab will begin Monday and his status is listed as "day-to-day."