USC quarterback Aaron Corp spent much of Tuesday wearing a slight grimace on his face and a rubberized sleeve on his swollen left knee.
Unable to practice because of an injury suffered the day before, Corp watched freshman Matt Barkley take command of the Trojans' morning walk-through and afternoon practice.
By evening, after doctors had read Corp's X-rays, examined him and indicated there was no ligament damage, the sophomore smiled easily and walked with a spring in his step rather than a limp.
"My pain tolerance is the only thing now, which is good news," Corp said.
The bad news, at least for Corp, is that Barkley continues to make a run at the starting job for the Trojans' Sept. 5 opener against San Jose State.
With Corp scheduled for an MRI exam to determine the cause of swelling, it seems unlikely that he will be able to practice this afternoon when the Trojans wear full pads for the first time.
That would provide another opportunity for Barkley, who said he "had a blast" taking the majority of snaps Tuesday, when Mitch Mustain also performed well in Corp's absence.
"I'm hoping he gets back as soon as possible," Barkley said of Corp, "but while he's out, I'm going to try and take advantage of every rep."
Coach Pete Carroll noted that Barkley is still struggling some with calling protections at the line of scrimmage, but the former Santa Ana Mater Dei High star otherwise has impressed.
"He's not shy a bit and he believes very much in his ability to move the club when he gets his chance," Carroll said.
Asked by a reporter if Corp could lose his hold on the No. 1 spot on the depth chart if he sat out a week, Carroll said, "I don't know why I would ever answer that question," adding that the Trojans needed to "get him back out here as soon as we can."
Carroll then invoked the name of a certain New York Yankees first baseman who was famously replaced by Lou Gehrig.
"Are you asking me if this is the Wally Pipp situation relived?" Carroll said.
When the reporter said yes, Carroll declined to answer.
Corp, though, was unfazed when asked if he were worried that Barkley could surpass him if he missed significant practice time.
"Not really," Corp said. "But I want to get out there."
Shane Horton is reveling in the switch from safety to linebacker, a move that was made to provide depth.
The 6-foot-1, 215-pound Horton is competing with projected starter Malcolm Smith and backup Jordan Campbell for playing time at the weakside spot.
"It's a little bit more physically demanding than playing safety, but the best plays I made last season were inside the box, so I like it," Horton said.
Tailback Allen Bradford had another long touchdown run. . . . Tailback Stafon Johnson said he did not practice because of a stomach virus. Fullback Stanley Havili (shoulder) also sat out. . . . The Trojans practice today at 3 p.m.