Lupul suffered the injury after taking a check from David Krejci in Tuesday's 5-4 loss to the Boston Bruins.
Forward Colby Armstrong was also injured in the game, breaking his nose in a fight with Dennis Seidenberg. He's listed as day-to-day.
It's possible Lupul may not see much more action this season with Toronto's final regular-season game slated for April 7.
The Leafs head into Wednesday's game in Pittsburgh five points back of the final playoff spot in the Eastern Conference.
Carter Ashton has been called up from the AHL's Marlies on an emergency basis.
Lupul is second in scoring for the Leafs with 25 goals and 42 assists in 66 games.