The forward left in the third period of a 4-3 shootout win against the New Jersey Devils on Sunday after his right leg bent awkwardly following a collision with Devils defenseman P.K. Subban.
The Rangers said he will need 6-8 months to recover, putting his earliest possible return in mid-May. The regular season is scheduled to end on April 29.
"Not good news for us," Rangers coach Gerard Gallant said. "Disappointing and tough, but it happens and you've got to move on. Nothing you could do. I just feel for the kid. It's a tough injury."
The 25-year-old had been skating as the right wing on the top line with Mika Zibanejad and Chris Kreider. He had four assists in 14 games this season.
"His physicality, his compete level is what we all want to play like, it's what our group has been striving to be," Rangers forward Barclay Goodrow said. "He's one of the hardest hitters I've ever seen, to be honest. He's a guy that people don't want to play against, and that's infectious throughout the group. It's a big hole to fill, but we'll get it done as a group."
The Rangers acquired Blais and a second-round pick in the 2022 NHL Draft in a trade with the St. Louis Blues for forward Pavel Buchnevich on July 23.
Blais has scored 39 points (17 goals, 22 assists) in 133 NHL games. He won the Stanley Cup with the Blues in 2019, when he scored three points (one goal, two assists) in 15 Stanley Cup Playoff games.
"His game was coming," Gallant said. "It's tough for a kid like him. He's starting to play, starting to get to know his teammates real well and obviously this happens and he's out for the year. You feel bad for the kid."
Filip Chytil replaced Blais when the Rangers defeated the Montreal Canadiens 3-2 on Tuesday; the center missed three games with an upper-body injury. Forward Greg McKegg was recalled from Hartford of the American Hockey League.
"[Blais] is the type of player that's built for the playoffs," Rangers forward Julien Gauthier said. "Hopefully we can get him back before that."
