Boucher wound up beneath teammate Ryan Parent and Bruins forward Miroslav Satan after a scramble for a loose puck with 15:25 left in the second period. The puck was cleared away, but Boucher remained on the ice; a referee whistled the play dead and quickly waved to the Flyers bench.
After the team's medical staff came out, backup Michael Leighton immediately stepped onto the ice in a ballcap and began stretching. A few minutes later, Boucher skated off with assistance, favoring his left knee as the Boston crowd gave him polite applause.
Flyers spokesman Zack Hill said that Boucher wouldn't return. The team didn't identify the injury.
Leighton was claimed on waivers from Carolina on Dec. 15 and became the primary starter when Ray Emery sustained a season-ending hip injury in February. Leighton won his first four starts before losing the Winter Classic at Fenway Park to the Bruins, 2-1 in overtime.
He had not played since spraining his ankle in Nashville on March 16, an injury that was supposed to keep him out about eight weeks.
Boucher, who started his career with the Flyers in 1999 but spent time with five other teams since then, filled in for him, going 9-18 in the regular season with a 2.76 goals-against average. He led Philadelphia in a five-game victory over New Jersey in the first round of the playoffs and had a 2.43 goals-against average in the playoffs before Monday night.
Philadelphia led the game 1-0 at the time of the injury, and extended its lead to 3-0 later in the second period. The Flyers needed a win to stay alive in the best-of-seven series, which Boston led 3-1.