Team doctors estimate he will need a four-to-six week recovery period.
Devereaux sustained the injury in the third period of the Leafs' 3-1 victory Thursday against the Columbus Blue Jackets. Devereaux has compiled 18 points (7 goals, 11 assists) with 22 penalty minutes and 79 shots on goal in 56games.
"I'd like to think it's not too bad but the tests will show us for sure," he said before Friday's MRI confirmed the ligament damage.
Because the Maple Leafs are last in the Northeast Division with only 59 points, good for 14th place in the Eastern Conference overall, the 10-year veteran may have played his last game this season.