USC defensive coaches moved Armond Armstead from tackle to end last spring, hoping the sophomore would give the line a larger presence on the edge.
Those plans went awry Thursday when the 6-foot-5, 295-pound Armstead suffered a broken bone in his left foot.
Armstead was injured during the morning workout when linebacker Shane Horton stepped on his foot.
He is scheduled to have surgery today and is expected to be sidelined six to eight weeks.
"I thought I was having a pretty good camp -- I thought I was getting better technique-wise playing strong-side end," Armstead said. "So it's pretty disappointing . . . but what's done is done. You have to move on forward."
Armstead's injury opens the door for sophomore Malik Jackson and redshirt freshmen Wes Horton and Nick Perry.
"My approach the whole time has been is to get everybody where we can play, so if something like this does happen the next guy can step in," defensive line coach Jethro Franklin said.
The 6-foot-5, 245-pound Jackson played in eight games last season. He had two sacks, a deflection and also forced a fumble.
Wes Horton, 6 feet 5, 255 pounds, has gained weight and strength since last season, and the 6-foot-3, 250-pound Perry is feeling more comfortable after several position switches.
"I'm getting the hang of it now," Perry said.
Incoming freshman defensive back Patrick Hall, who suffered a knee injury in his first practice with the Trojans on Wednesday, received more bad news when tests revealed a torn anterior cruciate ligament.
Hall will have surgery and is expected to sit out the season, a school official said.
Hall, who played at Ventura St. Bonaventure High, did not participate in workouts for the first 11 days of training camp because of academic certification issues.
Carroll said the Trojans worked out in the morning and afternoon without pads to "get more guys practicing and out here getting work by not banging it." ... Starting left guard Jeff Byers worked at center and Alex Parsons worked at guard to give the Trojans flexibility in the wake of Kristofer O'Dowd's knee injury. O'Dowd is expected to be sidelined two to three weeks because of a dislocated kneecap. Parsons will start in his place. . . . The afternoon practice ended when safety Taylor Mays intercepted a "Hail Mary" pass and returned it for a touchdown. . . . The Trojans practice today at 3 p.m.