USC's quarterback situation for the season opener and possibly beyond became muddled Friday when starter Mark Sanchez suffered a dislocated left kneecap while warming up for practice.
Sanchez, a fourth-year junior, threw a pass to teammate Clay Matthews, as he does every day before stretching exercises, but then crumpled to the ground.
Team trainers helped Sanchez to the sideline, where he said the kneecap was put back into place. He was later carted from the field and taken for an MRI exam and X-rays.
Sanchez, on crutches and wearing a long brace on his left leg, expressed disappointment but also relief after returning to campus and meeting with team doctors, who informed him there was no ligament, cartilage or bone damage. He will begin rehabilitation Monday and Coach Pete Carroll said the quarterback's status was day-to-day.
"Just scary, frightening," Sanchez said of his initial thoughts when his knee gave out. "But the way they said some of these things happen it could be a lot worse. This is really the best-case scenario."
Said Carroll: "We lucked out."
Sanchez, 6 feet 3 and 225 pounds, appeared in seven games last season, including three starts in place of injured senior John David Booty. He completed 69 of 114 passes for 695 yards and seven touchdowns with five interceptions.
Asked if he would be ready for the Trojans' Aug. 30 opener at Virginia, Sanchez said, "Hope so, got to hope so. And work to get back. But we'll see. I'm trying to get back before that so I can practice."
Sanchez's uncertain status elevates sophomore transfer Mitch Mustain and redshirt freshman Aaron Corp to compete for the starting job. Both appeared a bit shaky in the immediate aftermath of Sanchez's injury before rallying and performing well during the second half of practice.
Carroll said the injury provided an "extraordinary moment" for the Trojans' backups.
"My thought is we have a guy like Mitch and a guy like Aaron that can jump in and take their shots at this thing until Mark's back in here and battling with them," Carroll said. "This is an opportunity and how we deal with it and how we handle it is important. Not the fact that something happened."
Offensive coordinator Steve Sarkisian indicated Mustain was next in line but said Corp would also compete for the spot.
"These two guys are more on the athletic side -- they really like to get out of the pocket and do things out of the pocket," Sarkisian said.
Mustain was 8-0 as a freshman starter at Arkansas in 2006. On Thursday, Mustain had said he did not think it would be possible to unseat Sanchez as the starter for the opener, but that he would be prepared if needed.
Mustain called Sanchez's mishap a "freak" injury.
"We hate that it happened. We hate for it to happen like that, especially, but we got an opportunity today to kind of show what we could do and had to take care of it," he said.
Corp starred at Orange Lutheran High and was the standout performer during the Trojans' spring scrimmage in April. On Friday, he passed for three touchdowns during the team's final drill.
"This is an opportunity," he said. "I'm thrilled but at the same time you have to feel for Mark."