Lupul was injured Wednesday night in the second period of Toronto's 5-2 win at Pittsburgh. Lupul was in front of the net during a power play when he was hit in the arm by a slap shot taken by teammate Dion Phaneuf.
Lupul left the game immediately and didn't return.
The Maple Leafs say that Lupul was placed on the injured list. He was replaced on the roster by forward Matt Frattin, who was recalled from the Toronto Marlies of the AHL before the Maple Leafs hosted the New York Islanders on Thursday night.
Frattin has nine goals and eight assists in 21 games with the Marlies.
Lupul, who signed a five-year contract extension on Sunday, failed to record a point in three games this season.