Vanek, however, didn't rule himself out of the series tied at a game apiece.
Sabres defenseman Steve Montador expects the team to stick with the same system without Vanek.
"Thomas is hard to replace, but one player doesn't make a team, and you can say that about any one guy in our lineup," Montador said. "We want to continue to play hard, play our system and we'll get the chances that we need if we continue to work."
Buffalo's top goal scorer was injured after he was hooked and slashed by Boston's Johnny Boychuk during a partial breakaway with under 7 minutes left in the opening period of the Bruins' Game 2 win Saturday.
Vanek slid hard into the end boards and had difficulty putting weight on his left foot.
After the game, Sabres coach Lindy Ruff called the play into question by characterizing Boychuk's action as a "two-hander." After an off-ice workout Sunday in Boston, Boychuk said he had no intent to injure on the play.
"I was just trying to lift his stick on the one side because it was kind of like a breakaway," he said. "And then I couldn't get to it, so I went to the other side and tried to hit his stick. I didn't even know I actually slashed him."
In Vanek's absence, first-line center Tim Connolly becomes more important to the Sabres. Buffalo's second leading scorer, Connolly missed the last nine games of the regular season with a foot injury but played in both postseason games.
"I definitely can play better," he said. "The first couple of games I didn't get a lot accomplished out there, especially offensively.
"It's good sometimes to get on the road during the playoffs. You can really just go there and focus on playing the game. There's nothing else going on. I personally like playing on the road, especially in the playoffs."
Right wing Drew Stafford, meanwhile, will undergo tests Sunday to determine whether he can return for Game 3 on Monday. He has been sidelined with a concussion sustained in the second-to-last game of the regular season against Ottawa.
Information from Matt Kalman of ESPNBoston.com and The Associated Press was used in this report.