The 6-foot, 190-pound native of Muskegon, Mich., was injured during a scrimmage at the Los Angeles Coliseum on Saturday while catching a 34-yard pass from freshman quarterback Matt Barkley.
"It's just a crusher," Coach Pete Carroll told the Los Angeles Times about Johnson's loss.
He was USC's third-leading receiver in 2008, his sophomore season, with 33 receptions for 570 yards and eight touchdowns in 13 games.
He added 14 kickoff returns for 380 yards, a 27.1-yard average per return.
In Johnson's absence, fellow junior Damian Williams becomes the chief target of Barkley, who will be the first freshman ever to start at quarterback for the Trojans, the fourth-ranked team in the pre-season USA Today coaches poll.
Carroll has not named a replacement.