Just when freshman quarterback Matt Barkley made another stride forward, the USC offense took a big hit.

Starting receiver Ronald Johnson will be out six to eight weeks after breaking his collarbone during the No. 4 Trojans' mock game Saturday at the Coliseum, depriving Barkley of one of his top targets until at least mid-October.

Johnson was the Trojans' third-leading receiver last season with 33 catches for 570 yards and eight touchdowns. Coach Pete Carroll said the injury was "the hit of the hit of the weekend," and called Johnson's absence "just a crusher."

Johnson was hurt during the second series of the mock game when he was tackled after catching an underthrown 34-yard pass from Barkley, who went 10 for 17 for 159 yards with one touchdown and one interception on a tipped pass against the Trojans' scout-team defense.

Johnson dropped to the turf while running off the Coliseum field, and X-rays later revealed the break.

"We'll be OK," said Damian Williams, the Arkansas transfer who led the Trojans with 58 catches for 869 yards last season. "We've still got great guys there. We'll keep pushing, but we're definitely going to miss him."

Williams will take Johnson's spot at flanker, where Williams played last season. Carroll said junior David Ausberry will move into the starting lineup at split end, while promising freshmen Brice Butler and De'Von Flournoy also will move up the depth chart.

"I'm ready for it," said Butler, a Georgia high school star who had a team-leading four catches for 66 yards in the mock game. "Last year I wasn't ready. I thought I was, but I wasn't. But now I feel like I'm ready and more mature."

Although Barkley tripped and fell to the turf on his first snap in the mock game, he looked cooler and more collected in his second run-through with the first-team offense at the Coliseum, where he had a forgettable debut scrimmage earlier in the month.

Carroll selected the freshman to be his starting quarterback Thursday, pre-emptively ending his training camp competition with Aaron Corp before the mock game.

"It's a great feeling, but it didn't change the way I'm approaching anything or how I play the game," Barkley said. "It's great that the team supports me and they're behind me."

Barkley played a controlled, steady mock game, regularly finding empty spots in the defense and rarely making risky plays. Despite his opening-snap slip, he managed to pitch to tailback Joe McKnight for a 6-yard gain _ but he threw an interception on a tipped pass to end the first drive.

Barkley eventually threw a touchdown pass and ran for another score before giving way to Corp. Running back Stafon Johnson rushed for two scores, and Williams had two catches for 36 yards.

"I think I've seen every trick in the book from our defense," Barkley said. "I think we're better prepared than a lot of offenses are from what we see in practice, all the looks and fronts our defense can give you."

Corp also looked sharp for the Trojans in his late-game work, going 5 for 7 for 89 yards and moving smoothly on his left leg just three weeks after getting a small break in the bone below his knee. Easily moving inside and outside the pocket, Corp proved himself ready to play next week in the season opener against San Jose State.

"I said to him, 'When you finally take off and hook-slide, I'll know you're ready,'" Carroll said. "He did that, and he kind of turned and looked at me. He showed he's ready to play, and he's available for Saturday."