Starting center Kristofer O'Dowd, regarded as a linchpin for an experienced line expected to ease the transition of a new quarterback, suffered a dislocated right knee during the first series of a scrimmage Monday night at the Coliseum.
O'Dowd, a junior coming off shoulder surgery, went down at the end of a mistake-prone first series on a running play that ended with Joe McKnight's fumble.
Senior Alex Parsons replaced O'Dowd, who is expected to be sidelined three to four weeks.
"We're fortunate that we have depth, but it's just a crusher that he got banged up like that," Coach Pete Carroll.
As a freshman in 2007, O'Dowd also dislocated his right kneecap in the fourth game against Washington and missed four games.
Senior Nick Howell also suffered a sprained ankle, leaving the offensive line a bit in flux ahead of the Sept. 5 opener against San Jose State.
Barkley remains the front-runner to start the opener unless Aaron Corp returns from a leg injury in time to retake the No. 1 spot on the depth chart.
Corp, who has been out for seven days, threw passes to Carroll before the game but is still unable to run. He was scheduled to have X-rays on the cracked bone in his leg on Monday but will instead have them today.
Barkley, who had looked good in the Trojans' first scrimmage last week, once again took all of the first-team snaps, but he said he was "maybe a little too amped" at the outset.
Barkley's first pass downfield was "the worst throw he's thrown since he's been here. He just chucked it," Carroll said. Under pressure behind the line of scrimmage, he drifted to his right and threw into double coverage, barely avoiding an interception.
"The defense really played well in coverage," Carroll said.
Said Barkley, who completed only five of 18 passes for 109 yards: "It will be a good tape to watch and to learn from."
Junior Mitch Mustain completed 10 of 16 passes for 108 yards against the reserves. Mustain benefited from freshman De'Von Flournoy's 53-yard touchdown that came on a catch-and-run.
Flournoy finished with three catches for 105 yards.
McKnight rushed for 77 yards in six carries and Allen Bradford gained 51 yards in eight carries.
Safety Taylor Mays had six tackles. . . . Ronald Johnson's 99-yard kickoff return for a touchdown and McKnight's 78-yard punt return for a score both came against the scout team. . . . Safety Drew McAllister did not participate because of a concussion suffered in practice Sunday. Freshman Byron Moore moved from cornerback to safety to provide depth. . . . USC's 2012 game against Syracuse will be played at the new stadium at the Meadowlands. It plays host to Syracuse in 2011. . . . USC practices at 3 p.m. today.