USC quarterback Aaron Corp will be out one to two weeks after an MRI on Wednesday revealed a small crack on the head of his left fibula.
Corp was hurt Monday when a lineman rolled into his knee, though he missed only a few plays before returning to practice.
But Corp couldn't practice Tuesday because of pain and swelling. An X-ray was inconclusive but revealed no ligament damage. The injury will not require surgery.
"This is an unfortunate setback for Aaron but we're encouraged by the diagnosis and that he's going to return quickly," Trojans coach Pete Carroll said in a statement.
Corp said in the same statement that he remains optimistic about the situation.
"It's the best-case scenario," he said. "I'll keep rehabbing and hopefully be back soon."
It was eerily reminiscent of a knee injury former USC quarterback Mark Sanchez suffered on the exact same day of fall camp last season, which sidelined him for several weeks. Sanchez returned to start the season opener against Virginia.
"Mark's was a lot more serious," Corp said Wednesday. "But when I got hit and heard the pop, I was really scared. I thought it could be even worse."
Touted true freshman Matt Barkley will take over the first-team offense in his absence. Corp, a sophomore, emerged from spring practices as No. 1 on the depth chart, but Barkley has raised eyebrows with impressive skills and maturity.
"Matt's going to try and make the most of this opportunity," Carroll said.
The crack, in time, will heal and is far less damaging than if Corp had suffered ligament damage. He is allowed to throw, but not run and walks with a pronounced limp. He said he is working as hard as he can to stave off disappointment and "keep in the game mentally," which is something he learned from Sanchez, who used a golf cart last season to ferry him from one part of the field to the other, continually barking at his fellow quarterbacks.
There was speculation that Barkley would surpass Corp during the season when he learned the system better. The timetable may have been moved up as Barkley is taking all the snaps with the first team and performing well by all accounts.
"He is out here playing football with these guys like he's been here for a long time," Carroll said after Wednesday's practice.
The former Gatorade National Player of the year, Barkley enrolled at USC last January and promptly signed up for an upper-division sports communication class to help him better understand the workings of the sports world. He lifted weights in the Trojans' weight room with offensive linemen at 6 a.m. every day during winter conditioning.
"It's been my dream for quite sometime," he said of being the guy at USC. "It's now becoming a reality. I pray a lot about it. It's getting close, but that's no reason to lay off. I'm going to keep my head down and work at it."
He realizes he still needs to work on reading defensive coverages and becoming "100 percent on protections."
Wide receiver Damian Williams was towing the party line Wednesday, repeating over and over that Carroll never strips a player of his job because of injury. He insisted that Corp was still No. 1, but offered some insight on the difference between the two passers.
"Timing is more important when Matt's in there, because everything with him is a timing throw most of the time," he said. "With Aaron it's timing, but sometimes he's moving around and sometimes you've got to cut your route off a little quicker or make a little bit of adjustment when he's on the run. It might be a shorter ball. The technique and detail [between the two] is very different."
Corp's hindered mobility is where Barkley might gain the most ground. Corp is a scrambler and can make things happen with his feet, something Barkley has been working on. However, if Corp is limited, one assistant said 'Barkley's going to win the battle.'
ESPN.com's Ted Miller and ESPN's Shelley Smith contributed to this story.