The first-line left wing suffered the injury during an awkward fall in Montreal's 3-0 win in Winnipeg on Tuesday.
"It's bad luck, but it could have been a lot worse," Canadiens coach Michel Therrien said. "We'll take time to make sure he gets better, but it the meantime, it's not such bad news. It could have been worse."
The U.S. Olympic team hopeful has had a rough start to the season. He jammed a wrist in the opening game but missed only one game.
Pacioretty is known for his quick recuperation from injuries, but when asked whether he would be back for the Canadiens' next game Saturday against Nashville, Therrien smiled and said, "This time, I don't think so."