Gustin will undergo surgery Thursday to repair the tear. Coach Clay Helton said there is no ligament damage to Gustin's knee.
"Usually that is anywhere from a two- to four-week process, sometimes five, depending on people, how they heal," Helton told reporters Wednesday. "We'll get it done [Thursday], have it cleaned up."
Gustin battled bicep and toe injuries in 2017, which limited him to only four games. He started every game in 2016, recording a team-high 13 tackles for loss and 68 total tackles. The senior has 109 career tackles, including 23 for loss and 14 sacks in 31 games, making 17 starts.
USC opens the season Sept. 1 against UNLV in Los Angeles, before traveling to Stanford for the Pac-12 opener on Sept. 8. Helton said former defensive lineman Rasheem Green had a similar procedure last season and returned in two weeks, while cornerback Ajene Harris had the same surgery this spring and returned in three-and-a-half weeks.