Gustin was injured during Tuesday's practice and had to be carted off the field. He will undergo surgery after an MRI exam revealed the damage to be a partially torn ligament, the Orange County Register reported, citing a source.
"He was not in a lot of pain," Trojans coach Clay Helton told reporters after Tuesday's practice. "It felt funny to him."
Gustin was limited to four games as a junior last season as he dealt with a fracture toe and torn biceps. He had three sacks in his first three games before the injury bug hit.
The 6-foot-5, 255-pound Gustin has appeared in 28 games in three seasons with the Trojans, recording 14.0 sacks and 23 tackles for loss.