The source says Farmer will redshirt and miss the entire 2013 season. He is expected to undergo surgery to repair the damage in two weeks.
The injury happened when Farmer was running a route during non-contact drills Tuesday. He fell to the ground and laid there for about five minutes before the training staff carted him off.
A source said there was no initial swelling following practice, but that Farmer would undergo an MRI to see if there was internal damage.
This is just just the latest setback for the Gardena (Calif.) Serra five-star product during his Trojan career. Farmer was a Scout.com No. 3 ranked receiver nationally. He was the high school teammate of both Robert Woods and Marqise Lee.
Farmer was plagued with injuries both his freshman and sophomore seasons, missing multiple games each year.
Through the first three weeks of spring practice the junior looked healthy and poised to earn a spot in the rotation. USC head coach Lane Kiffin said he was starting to make the "big plays" necessary to earn a role.
Most of the weight by the receiving corp during the 2013 season is expected to be carried by Biletnikoff Award winner Marqise Lee and sophomore Nelson Agholor. Now with Farmer out, the third spot in the rotation seems to be wide open for senior De'Von Flournoy, redshirt sophomore Victor Blackwell, and freshman Darreus Rogers to fill.