Thomas appeared to go down awkwardly when Sean Couturier wound up for a shot from just inside the Panthers' blue line. He stumbled trying to get back to his feet, and never got into position to stop Braydon Coburn's floating shot from the right side that gave the Flyers a 2-0 lead.
Thomas left the ice immediately and did not return to the bench. Jacob Markstrom replaced him and did not allow another goal, but the Flyers held on for a 2-1 victory.
"Just slightly pulled his groin, so I don't think it's anything long-term," Panthers coach Kevin Dineen said after the game. "But we will evaluate him [Wednesday] and see where he's at."
With the Panthers playing back-to-back games Thursday and Friday against the Tampa Bay Lightning and Pittsburgh Penguins, Dineen said the team could recall goalie Scott Clemmensen from the club's American Hockey League team, the San Antonio Rampage.
Follow Adam Kimelman on Twitter: @NHLAdamK