Sanchez injures his right thumb

USC backup quarterback Mark Sanchez could sit out the season opener because of a hairline fracture in the thumb of his throwing hand.

Sanchez said Monday that he injured a bone in his right thumb when his hand struck another player’s helmet after he threw a pass in Sunday night’s scrimmage at the Coliseum.

Sanchez, who was No. 2 on the depth chart behind John David Booty, said doctors told him he might be out six weeks. The Trojans play Idaho on Sept. 1 at the Coliseum.

“I don’t think it will be that long, but we’ll see,” Sanchez said.

Fifth-year senior Michael McDonald was elevated to Booty’s backup, but freshman Aaron Corp took the majority of second-team snaps and will continue to do so during the next few days, Coach Pete Carroll said.

Corp completed all four of his passes and connected with freshman receiver Ronald Johnson for a nine-yard touchdown in Sunday’s scrimmage.

“This is a tremendous opportunity for [Corp] to really step into it and see how far he can take it,” Carroll said.

McDonald, a former walk-on, is prepared for whatever role is required.

“I’ll just do what they tell me -- I’m ready to play if they need me to,” he said.

Sophomore Garrett Green is the fourth quarterback.

Sanchez’s injury occurred a few days after Carroll and offensive coordinator Steve Sarkisian counseled the third-year sophomore about trying too hard. The time away from competition on the field could be beneficial, Sarkisian said.

“Hopefully, it’s an opportunity to step away a little bit and watch John David, how he handles situations on and off the field, whether it’s the huddle, audibles or even dealing with the media,” Sarkisian said. “There’s a lot of things he can take a look at.”

USC defensive end Kyle Moore was all smiles, at least until Monday.

After adopting a hybrid 3-4 defensive scheme last year, the Trojans are back to a 4-3 alignment and Moore is regarded as the front-runner to open the season as a starter.

But Moore and heralded freshman Everson Griffen, who is pushing Moore for playing time, had to be separated by teammates during practice.

Moore, a junior from Georgia, compared the incident to a fight between brothers. He said the incident began when he razzed Griffen about back soreness in the same way Frostee Rucker teased him two years ago.

“I was messing with him a little bit, just me being the older guy telling him everybody’s out here hurting just like he is. We have to do the same drills he does. He has to suck it up and get through that, and he didn’t like that,” Moore said.

The players later reconciled, Moore said.

Griffen was unavailable for comment because USC has not made freshmen available to the media.

Senior defensive tackle Sedrick Ellis and other players and coaches said fierce competition, the hot weather and fatigue probably contributed to the blowup. “I’m sure they’ll be the best of friends by this afternoon,” Ellis said.

All-American left tackle Sam Baker did not practice because of rib-area soreness, Carroll said. Charles Brown, who appears on track to possibly start at right tackle, worked in Baker’s spot. Thomas Herring worked at right tackle. Running backs Emmanuel Moody (knee) and Joe McKnight (neck), linebacker Brian Cushing (general soreness) and receiver Vidal Hazelton (groin) were among other players who did not practice. . . . Defensive lineman Trey Henderson is scheduled to have arthroscopic knee surgery today. . . . USC’s tentative practice schedule for the week with all workouts on campus and open unless otherwise noted: Today 9 a.m. and 4 p.m.; Wednesday 3:30 p.m.; Thursday 10 a.m. and 7:15 p.m. (at Coliseum, closed to the public); Friday 3:30 p.m.; Saturday 9 a.m. and 4 p.m.; Sunday 3:30 p.m.

gary.klein@latimes.com