He suffered the injury in the second period of Wednesday night's 5-2 road win over the Pittsburgh Penguins. Lupul was in front of the net during a power play when he was hit in the arm by a slapshot from teammate Dion Phaneuf.
Lupul left the game immediately and did not return.
Lupul was placed on the team's injured reserve list and forward Matt Frattin was recalled from the Toronto Marlies of the American Hockey League, the Leafs said in a release before Thursday's home game against the New York Islanders.
Frattin has 17 points (9-8) in 21 games for the Marlies this season.
Lupul, who signed a five-year contract extension last Sunday, recorded 25 goals and 42 assists last season and made the all-star team for the first time. He did not record a point in three games this season.