Spring scrimmage No. 4 report

LOS ANGELES -- Tre Madden is no longer the story of spring practice for the USC Trojans. The linebacker-turned-running-back who caught fire after moving to offense last month tore his left knee on Thursday and is out for the 2012 season.

Now, the story is who's going to replace him.

Redshirt freshman Buck Allen, one of the most unknown quantities on the Trojans' roster, started to make his case in Saturday's scrimmage at the Coliseum with the most extended time he has experienced this spring while bothered by a right hamstring pull. USC coach Lane Kiffin indicated he has been giving a lot of thought to other options to fill the big-back role Madden had quickly assumed behind starter Curtis McNeal.

And, it's a bit crazy, but Kiffin also called Madden "one of the most valuable guys on the team" because the Trojans "don't have anybody else like him."

It really was a fast rise and fall for Madden this spring. It still has been only 20 days since he found out from the coaching staff he was being moved to running back for experimental purposes and less than two weeks since Kiffin confirmed he was staying there long-term.

Madden has 226 pounds on his 6-foot frame. Allen, his natural replacement, is listed at 215 pounds -- so there's ground to make up there. And he also has been bothered by injuries on-and-off since his arrival on campus last August. But he's really the Trojans' best hope behind McNeal and speedy No. 2 back D.J. Morgan.

"He's not completely healthy," Kiffin said Saturday of Allen. "But at least he's out here."

Allen looked a step slow and struggled with some of the plays, memorably going in the wrong direction on his first snap. He said he knew his hamstring was only about 85-90 percent healed but he pushed himself to return Saturday in Madden's absence anyway.

"I knew I had to step up because my teammate went down," Allen said afterward. "I could have take the day off, but I knew I had to step up for my team."

Kiffin said incoming athlete Nelson Agholor, a 2012 signee, is going to get time at running back this fall. And Jahleel Pinner, a fellow 2012 signee who the Trojans recruited as a fullback, also could get an opportunity.

There are no other possibilities. Kiffin said he has examined the rest of the USC roster for any other players who he could try out there and didn't come away with any answers.

Madden will redshirt the season and presumably be in position to participate in spring practice for the 2013 season. But he may be limited then, too. Morgan tore ligaments in his knee in high school and has maintained it took him roughly two full years to get back to how good he fault pre-injury.

But one thing's for sure: Madden won't be going back to linebacker, where he was looked at as an eventual starter before the position switch this spring. His short time on offense was too impressive for the Trojans to scrap their plans.

"He's definitely a running back," Kiffin said Saturday.

Final notes: Receiver De'Von Flournoy did not participate in practice because of an ankle injury suffered earlier this week, and tight end Junior Pomee had to come out when he re-aggravated the foot injury he suffered last week. Safety Drew McAllister returned after missing Thursday's session. ... Marqise Lee had his worst day of the spring, dropping multiple passes and showing his frustration by slamming his arms on the grass after each mistake. Kiffin called it "very unusual for him." ... Quarterback Jesse Scroggins (hip) did individual drills after missing the entirety of Tuesday and Thursday's sessions to focus on academics.