The 29-year-old sustained the injury during training on Thursday and was immediately assessed by the club's medical staff.
Farnerud's worst fears were confirmed, as the club confirmed that he had ruptured the anterior cruciate ligament in his left knee.
The Sweden international will undergo surgery in the coming days, after which the club will be able to estimate how long he will be out of action for.
It comes as a blow to the 10th-placed Torino and the player himself, who has scored four times this season, the last of which set up the 2-1 victory at Catania on Sunday.
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