"We will know more on this within a day or two as we await results of tests," Holmgren said in announcing the injury.
Hartnell saw 17:13 of ice time in the Flyers' 3-0 loss at the New Jersey Devils on Tuesday. He has one assist and 13 penalty minutes in three games this season.
The Flyers will be without Hartnell, Brayden Schenn and Zac Rinaldo for Thursday's home game against the New York Rangers. Schenn was suspended by the NHL for one game for a hit on New Jersey defenseman Anton Volchenkov in Tuesday's game, and Rinaldo is out with a lacerated right thigh.
I don't know how he does it. I don't know how he gets his body parallel with the player and pulls it through his legs like that. I know he's tried it a couple times in practice and it's never worked, so how he does it in a game, it's incredible.
— Capitals defenseman Mike Green on teammate Alex Ovechkin's highlight-reel goal against the Devils on Saturday