Fleury was placed on injured reserve after sustaining a concussion in a 6-3 loss to the Detroit Red Wings on Friday. He played in each of the Golden Knights' first four games, going 3-1-0 with a 2.48 goals-against average and .925 save percentage.
Forward Jonathan Marchessault was also placed on injured reserve with a lower-body injury.
Rookie Malcolm Subban, claimed on waivers by the Golden Knights from the Bruins on Oct. 3, is expected to start against his former team. Vegas recalled goalie Maxime Lagace and forward Alex Tuch from Chicago of the American Hockey League on Sunday.
Center Vadim Shipachyov, who signed a two-year contract with Vegas on May 4, is expected to make his NHL debut in place of center Erik Haula, who will miss at least a week because of a lower-body injury.
The Bruins, who won 6-2 at the Arizona Coyotes on Saturday, are again expected to play without center Patrice Bergeron. He has yet to dress for a game this season because of a lower-body injury.