That doesn't bode well for the Phoenix Coyotes, who have had enough trouble in the series as is.
Vanek is expected to be in uniform as the visiting Canadiens go for a 10th consecutive win over the Coyotes on Thursday night.
Montreal (35-22-7) made a big splash Wednesday, sending prospect Sebastien Collberg and a conditional 2014 second-round pick to the Islanders in exchange for Vanek and a conditional fifth-rounder. The picks will only be exchanged if the Canadiens make the playoffs.
Vanek, who opened the season with Buffalo before getting sent to New York on Oct. 27, becomes Montreal's leading scorer with 53 points.
"When you think of a hockey Mecca, Montreal is it. I played many times for Buffalo in Montreal and it's so electric. I'm super excited," he said. "It's been a tough year for me and my family. I knew there was a chance we'd move again. Now it's over and I'm a Montreal Canadien and I can just focus on hockey and helping that team win a lot of games."
The Canadiens occupy second place in the Atlantic Division with 77 points.
"The most important thing is to make the playoffs and right now I have a chance to do so. This is why we play," Vanek, who has two goals and six assists over an eight-game stretch against Phoenix, told the Canadiens' official website. "Anything can happen in the playoffs."
Montreal improved to 6-1-1 in its last eight after pulling out a 4-3 shootout win at NHL-leading Anaheim on Wednesday. Captain Brian Gionta and Max Pacioretty both scored, and Dustin Tokarski made 39 saves in his season debut while starting in place of the injured Carey Price.
"We knew we were going to play against the best team in the league, and they especially play great at home," defenseman Andrei Markov said. "So we just tried to stick to the game plan and our system and do our job.
"Everybody played hard, and our goalie played unbelievable. He made great saves and was really in the game."
The Canadiens will try to stay hot against the Coyotes, whom they have outscored 34-17 during a nine-game winning streak. Pacioretty scored twice and added an assist as Montreal took this season's first matchup 3-1 at home Dec. 17.
The Canadiens are 11-0-0 with three ties in the series since falling 4-2 at Phoenix on Dec. 9, 1998.
The Coyotes (28-23-11) snapped a season worst-tying four-game losing streak with Tuesday's 1-0 victory over Vancouver. Antoine Vermette scored his team-leading 22nd goal and Mike Smith turned away 23 shots.
"It was a pretty boring game from my standpoint, but I think that's a good thing," said Smith, 5-3-0 with a 1.64 goals-against average and three shutouts in his last eight home starts.
"We all know how close it is and to beat a team right there with us in the standings is big."
With 67 points, the Coyotes are one behind Dallas for the Western Conference's second wild-card spot. They traded for veteran forward Martin Erat on Tuesday in their only noteworthy move before the deadline.
"We like our team," general manager Don Maloney told the team's official website. "Adding Erat was a key for us. If we can stay healthy and get Martin Hanzal back healthy, that really sets our forward group well. Defensively, we are where we are. Hopefully it's good enough. And if Mike continues to play the way he did (Tuesday) night we'll be in great shape here."
Hanzal is questionable after sitting out against the Canucks with a lower-body injury.