Coach Peter Laviolette announced the decision prior to the game, saying he liked how Boucher "bailed us out of a couple of situations" in a 5-4 win Saturday.
The series is tied at one game apiece.
Boucher stopped 20 of 21 shots in taking over for Bobrovsky, who was pulled after allowing three goals on seven shots 12:30 into the game.
Boucher is the Flyers' most experienced goalie. He has an 18-15 career playoff record. Bobrovsky went from being the Flyers starter to a healthy scratch as Michael Leighton served as the backup.
The Flyers were without forward Andreas Nodl, who sustained an upper body injury Saturday. He was replaced by Nikolay Zherdev.
Sabres defenseman Jordan Leopold returned after missing 10 games with a broken bone in his left hand. He finished the regular season ninth on the team with 35 points, and led Sabres defensemen with 13 goals.
Leopold replaced defenseman Andrej Sekera, who played Game 2 after missing the opener with an upper body injury.