"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.
We don't have any excuses tonight. Excuses are for losers. We've played with five defensemen before and it didn't affect us. We just had a bad second period and it cost us the game. We have to look in the mirror and blame ourselves.
— Bruins coach Claude Julien on his team's play with the loss of defenseman Johnny Boychuk