The injury occurred during a non-contact drill on Tuesday.
The surgery -- his second on the knee -- will cause Dennis to miss the 2006 season.
"He was crushed by the whole thought of it, going through this again and all," Coach Pete Carroll told the Los Angeles Times. "But by the time we finished talking he had already started to turn his sights toward the goals he'll set up for himself for recovery."
Dennis, a fifth-year senior, sat out the 2004 season because of a similar injury. He was expected to be the Trojans' top running back next fall after the departures of Heisman Trophy winner Reggie Bush and LenDale White.
Dennis is expected to appeal to the NCAA for a sixth year of eligibility.
Notable deaths of 2014 [PHOTOS]
Michael Sam cut from the Dallas Cowboys