USC quarterback Mark Sanchez dislocated his left kneecap before practice Friday and will be sidelined for an unknown amount of time, which means he might miss the Aug. 30 season opener at Virginia.
Sanchez hurt himself when he landed awkwardly while throwing a pass before practice to linebacker Clay Matthews. There were initial fears he tore knee ligaments, but an MRI showed no damage, Sanchez said.
"It could be a lot worse," Sanchez said. "This is the best-case scenario. They didn't give me a time frame. It could be a short period or a long period. Hopefully it's the shorter period."
Asked he would play against Virginia, Sanchez said, "I hope so. We'll see."
Officially, USC lists Sanchez as day-to-day. Trojans coach Pete Carroll said sophomore Mitch Mustain, a transfer from Arkansas, and freshman Aaron Corp will split practice time. Mustain was ahead of Corp in the competition to be Sanchez's backup before Sanchez got hurt.
"We got lucky," Carroll said. "There are no other complications. It's a day-to-day deal. We'll wait and see. It will be immobilized over the weekend, and then he'll start rehab on Monday. It's day-to-day and we'll see how it responds."
Carroll said it was too early to tell who might start if Sanchez is unable to play.
"What it means for us is that it's an extraordinary moment for the other two quarterbacks," Carroll said. "Mitch has played a lot of games (at Arkansas), but it's not that valuable in some regards because it's not our system. I'm more concerned about having only two quarterbacks."
Garrett Green, who played quarterback at Notre Dame High in Sherman Oaks but is currently a wide receiver, is expected to move back to quarterback today. The coaches also are considering moving wide receiver Brandon Carswell, who played quarterback in high school.
Mustain and Corp said they were confident they could fill the starting role, after each expressed skepticism earlier in the week that there was much chance of beating out Sanchez.
"I think it changes (things) a little bit," Mustain said. "This is a perfect couple weeks for us to respond. That's why we're here. I don't think leadership will be a problem. I've played quarterback before."
Mustain said Friday's practice felt different once he and Corp were thrust into more prominent roles.
"I'm sure Mark felt like that when (John David Booty) got hurt (last year)," Mustain said. "It's a natural response."
Corp threw three touchdown passes during Friday's practice and said he would be prepared to start.
"It's an opportunity. I'm thrilled, but at the same time I feel for Mark," he said.
Also: Freshman wide receiver Brice Butler sprained his ankle and wore a protective boot on his foot. ... Offensive tackle Tyron Smith will be out at least a week with a sprained ankle. ... Freshman defensive tackle Jurrell Casey practiced with the starting defense Friday and Carroll said he will compete for playing time this season. "He looks very much like (former All-American) Mike Patterson when he got here," Carroll said. Casey currently weighs 300 pounds after weighing 270 in high school.