The feisty forward — who has given the Habs a spark with five goals and three assists already this season — left the game in the third period after taking a hit from Flyers defenceman Luke Schenn.
Canadiens head coach Michel Therrien says Gallagher is already "feeling a lot better," though he stressed that the club will follow league protocol and make sure he's fully recovered.
"We never want to put a player's health in danger," Therrien said following Sunday's practice. "When he gets back in the lineup, he will be healthy."
Under the NHL's new rules this year, a player who suffers a concussion no longer has to remain inactive for seven days, but must pass a series of tests. Gallagher had one concussion while playing junior.
When asked about whether Schenn's hit should get a second look by the league, Therrien said it's not for him to decide.
"I honestly hope that all bodychecks like this are reviewed," he said. "I have nothing else to add."
Gallagher had a goal and an assist against the Flyers before getting hurt, helping the Canadiens to their third straight win.
Forward Max Pacioretty, who also left Saturday night's game after getting cut by a skate as he drove to the net, was treated with stitches and was back at practice Sunday.
Pacioretty has a history of quickly returning from injuries, but he said he wasn't feeling 100 per cent and would see how he felt on before deciding if he can play when Montreal hosts the Carolina Hurricanes on Monday night.
Defenceman P.K. Subban, who is out with the flu, and Gallagher were the only players missing from Sunday's practice. Subban could also sit out of Monday's matchup against Carolina.
Carey Price, meanwhile, is feeling better after his own bout with the flu, but backup Peter Budaj could still get the start in goal against the Hurricanes.
The Canadiens travel to New York to face the Rangers on Tuesday. Goalie Robert Mayer, who was called up from the Hamilton Bulldogs on Saturday to back up Budaj, was sent back down to the American Hockey League.