a closer look LOS ANGELES That three-way quarterback battle the University of Southern California went through in the spring to decide a successor for John David Booty may have seemed like a bit of a pain at the time.

After all, the Trojans have had a mere three No. 1 quarterbacks over the past eight seasons, and two of them, Carson Palmer and Matt Leinart, went on to win the Heisman Trophy.

But now that Mark Sanchez, the three-year veteran backup and designated No. 1, has gone down with a freakish dislocation of his left kneecap, the problem of too many quarterbacks has turned into the promise at the position.

USC has moved on in the past few days, perhaps faster than any other program could, almost without missing a beat. Redshirt sophomore Mitch Mustain and fast-rising redshirt freshman Aaron Corp are battling to be the starter, should Sanchez not recover in time to play in the Aug. 30 opener at Virginia.

It's a far cry from last year, when Booty broke a finger in the sixth game and USC didn't have enough confidence in the seldom-used Sanchez to put him in. Booty ended up throwing four interceptions in the stunning 24-23 loss to 41-point underdog Stanford.

Now, having spread the reps around in the spring competition, the Trojans have two backup quarterbacks who are capable of stepping into Sanchez's starting role.

"An enormous opportunity," is the way USC coach Pete Carroll describes it.

Or, as one poster on a Virginia fan site noted, "Great, only Virginia could be unlucky enough to have USC lose their No. 1 quarterback with a 2-1 record as a starter and bring in a guy who was 8-0 in the SEC."

That would be Mustain, who went 8-0 as a true freshman starter at Arkansas in 2006. While he wasn't high on his efforts of the previous two days, both he and Carroll agreed that things went well at Monday's morning session, when USC installed its two-minute drill and went no-huddle for the first time. The afternoon featured the installation of the goal line and short-yardage games

There also was a practice-ending chance to work on a "Hail Mary" play, earlier than usual for USC, and Mustain completed it for a touchdown at the buzzer.

"You noticed who Mitch threw it to," Carroll said. None other than high-leaping Damian Williams, who also played with Mustain at Springdale (Ark.) High and Arkansas.

The quick-footed Corp has had plenty of moments, as well. He's also hooked up with a one-time high school teammate, true freshman Blake Ayles, on a number of plays and has managed to stay "neck and neck" with Mustain, Carroll said.

But while Sanchez was named the starter before the end of spring so he could assume the leadership role, don't look for USC to make a decision on a new No. 1 right now. For one thing, Sanchez, who started his knee rehab Monday, has an outside shot at making it back in time for the opener.

But there's another reason, as Carroll pointed out with a smile: "We don't have to."