"A meeting will take place on Tuesday to decide if Fred needs surgery, which would mean he will not be available for three months, or if the injury can be treated through a period of complete rest which could last for four months".
The 24-year-old flyhalf, who has won 37 Heineken Cup caps, suffered his injury last Saturday in a Top 14 match which was his first game in two months because of a previous knee injury.
Michalak is yet to feature in the Toulouse Heineken Cup side this season. The last match the two-time Heineken Cup winner played at European level was the 2006 quarter-final, when Leinster produced a memorable victory over Toulouse in the south of France.
France coach Bernard Laporte omitted him from his November squad.