Goaltender Mike Smith, the leader of the charge into the battle for a playoff spot, was injured making a save and had to be helped off the ice with a lower body injury.
"Arguably, he's been our MVP," Calgary coach Glen Gulutzan said. "But he's a guy that takes such good care of himself and we're hoping this will be minor. But if it isn't, we've got other guys that can come in. It's what other teams have had to do this year."
Rookie David Rittich will start for the Flames on Tuesday night against the Boston Bruins at TD Garden. There was no word on the severity of Smith's injury, but he wasn't going to start against the sizzling Bruins anyway. The veteran is 3-12-0 with a 3.32 goals-against average and .887 save percentage lifetime against Boston.
Smith was not at practice at the Bruins' facility on Monday.
The Bruins (34-12-8) had their 18-game points streak (14-0-4) stopped by the lowly Buffalo Sabres at home Saturday night but are 19-2-4 in their last 25 games. Tuukka Rask, who lost that game, is 19-1-2 in his last 22 starts.
Rask is 4-0-3 with a 1.61 GAA and a .940 save percentage lifetime against the Flames.
The Flames, who will play the fifth game on a six-game road trip, are sitting in third place in the Pacific Division, two points behind the San Jose Sharks and three ahead of both the Los Angeles Kings and Anaheim Ducks.
Calgary (29-19-8) recalled goalie Jon Gillies on an emergency basis Monday -- and Gillies gets to return to the site of his NCAA title games for Providence in 2015.
Rittich, a nifty 5-1-2 with a 2.20 GAA and .927 save percentage, will start, according to the original plan.
The Bruins rebounded from Saturday night's debacle by taking four different leads and winning 5-3 at New Jersey on Sunday. They had two short-handed and one power play goals in the game.
"Penalty kill was real good," Boston coach Bruce Cassidy said. "They had some looks with the amount that we had to kill. We got a big power-play goal to help us and (two shorties) as well. Ends up being the difference in the game.
"Normally, we're a very good five-on-five team, so we're fine with that game, but our special teams -- tip of the hat to the guys out there."
The Bruins and Flames split two games last year, each winning on the other's ice.
Former Boston College standout Johnny Gaudreau is on a hot streak for the Flames, with two goals and five assists in the last four games. He was named the NHL's second star of the week on Monday.
Before the game, the Bruins will honor the 40th anniversary of the 1977-78 Stanley Cup finalists -- the only team in NHL history with 11 20-goal scorers. Don Cherry, the coach of that team and still a Canadian television star at age 84, will join the players in the ceremonial first puck drop.
The 11 are Peter McNab (41 goals), Terry O'Reilly (29), Bobby Schmautz (27), Stan Jonathan (27), Jean Ratelle (25), Rick Middleton (25), Wayne Cashman (24), Gregg Sheppard (23), Brad Park (22), Don Marcotte (20) and Bob Miller (20).