"This is very unfortunate news about Jabari, but I know he will rehab well and come back strong next year," Trojans coach Steve Sarkisian said. "And I know our team will step up in his absence."
The injury happened during a Sunday morning practice after Ruffin -- a redshirt sophomore -- intercepted a pass and was making a return. He was tackled and stayed on the ground before being helped off the field by trainers.
Sarkisian described the injury in his post-practice media session as a "knee tweak" and said he would know more Monday, but the diagnosis was worse than expected.
Ruffin (6-foot-3, 240 pounds) was playing the best football of his young career in the opening week of fall camp and was ready to be a major contributor in the rotation at the strongside linebacker spot, where he was locked in a fierce position battle with Quinton Powell. Ruffin also was expected to be a factor on special teams for the Trojans.
With fewer than 70 scholarship players in the final year of NCAA-mandated roster restrictions, the loss of a potential starter is a tough blow for the Trojans. The USC defense already was without reserve nose tackle Kenny Bigelow, who is out for the season with a knee injury suffered in summer workouts.
Ruffin had one start in 2013 with 22 tackles.
True freshman Uchenna Nwosu had been the third-team performer at strongside linebacker and could be moved to the backup spot behind Powell.