Linesman Brad Kovachik left Game 2 of the Eastern Conference Final on Monday in the second period with a sprained knee after a collision along the boards.
The NHL said he will be out indefinitely.
Tampa Bay Lightning forward Steven Stamkos checked New York Rangers defenseman Kevin Klein, who hit Kovachik with 7:41 left in the period.
Kovachik, who was helped off the ice, was replaced by Greg Devorski. The game was delayed about five minutes.