Lupul, who recently signed a five-year, $26 million contract, was injured during Wednesday's win over the Pittsburgh Penguins when a shot by team mate Dion Phaneuf struck him in front of the net.
"He's a huge part of our team up front, and it's terrible when you lose a guy like that," Phaneuf told reporters.
Lupul was scoreless in three games but ranked second among the Maple Leafs in goals (25) and points (67) last season.
