The Los Angeles Times initially reported the news on Wednesday night.
Farmer was participating in a noncontact drill while running a route when he planted his leg and crumpled to the ground. He was attended to immediately by trainers and several teammates. An MRI on Wednesday revealed the tears.
Farmer was having the best spring of his USC career after battling an assortment of injuries in his first two seasons. By all accounts, he had solidified the role of No. 3 receiver behind starters Marqise Lee and Nelson Agholor.
Farmer has career totals of five catches for 49 yards, five carries for 15 yards, three kickoff returns for 59 yards and two tackles on special teams.
Farmer came to USC as a member of the 2011 recruiting class, when he was ESPN's No. 1-ranked receiver in the nation. He played at Gardena (Calif.) Serra, where he was a member of a state championship team along with future USC teammates Lee and Robert Woods.
The team also announced that early enrollee Su'a Cravens, the No. 12 player in the 2013 ESPN 150, will have surgery for a torn meniscus and will miss four to six weeks.