Coach Michel Therrien said Pacioretty saw the team doctor on Wednesday and has an upper body injury, and that his condition will be evaluated next week.
Pacioretty fell against the boards after a check from Dmitry Kulikov early in Montreal's 4-1 victory in Florida on Sunday. He may have suffered a concussion, but it could also have been a neck or shoulder problem.
Therrien would not elaborate.
"We know exactly what he's got," the coach said. "We have to protect the players.
"That's the most important thing for us. There are things that we can't tell (you). We're a week away from the playoffs. There's a possibility that Max will play next week. It's an upper body injury."
He will sit out a home game Thursday night against Detroit and the regular season finale Saturday night in Toronto.
Pacioretty has been Montreal's best and most consistent forward this season. He leads the club with 37 goals and 67 points. The New Canaan, Conn. native has also developed into a strong two-way player and penalty killer. His plus-38 leads the NHL.
"It's a big loss, no doubt," said Therrien. "He's a player who gets quality ice time, not only offensively but defensively as well.
"We use him at the end of games if we're protecting a lead and on penalty-killing. He'll miss these two games and it will give someone else a chance to show what they can do. We reacted well when we lost him against Florida. We were able to create chances and we scored four goals."
In practice on Wednesday, Alex Galchenyuk was in Pacioretty's spot on left wing of the top line with Tomas Plekanec and Brendan Gallagher. Devante Smith-Pelly was promoted to the second unit with David Desharnais and Pierre-Alexandre Parenteau.
The Canadiens don't have a captain, but Pacioretty is one of four alternates who share the leadership with Plekanec and defencemen Andrei Markov and P.K. Subban.
"Leadership is amongst our whole group, not just the captains," said Subban. "Now we need guys to step up.
"That's OK, every team goes through that."
The Canadiens (48-22-10) hold a slim two-point lead over Tampa Bay for top spot in the Atlantic Division, with each team having two games to play.
The Canadiens announced that versatile forward Dale Weise was the winner of the Jacques Beauchamp Trophy as the team's unsung hero in voting by local media. Weise, with 10 goals and 29 points in 77 games, has been used on the top line and in checking roles this season.
"I'm extremely proud of it," said Weise. "I base my game on working hard and I hope the fans appreciate it."