Coach Alain Vigneault would not disclose the nature of McDonagh's injury before the Rangers game with the Tampa Bay Lightning, referring to a team statement that said the 26-year-old defenseman would be out indefinitely with an upper-body injury.
McDonagh, the team's top defenseman and a key component in all the special teams, has appeared in 73 games with nine goals and 25 assists. His plus-26 rating is the best on the team.
"Injuries are a part of the NHL game," Vigneault said. "When Ryan was out and Nasher (Rick Nash) was out and Henrik (Lundqvist) was out, it was about finding ways to win."
That's what the Rangers will have to do now, knowing the playoffs are going to start in the middle of next week.
McDonagh missed four games with a concussion in February after being punched by Wayne Simmonds of the Flyers. He missed two more games later in the month after being elbowed by Maple Leafs forward Leo Komarov.
McDonagh has never missed a playoff game, but he was limited in the Eastern Conference final against Tampa Bay last season after suffering a broken right foot in the series.
New York recalled defenseman Brady Skjei from AHL Hartford on Tuesday. Vigneault said the 2012 first-round draft pick will play in the final three games and see duty on the second power-play unit, where McDonagh normally plays with Dan Boyle.
Boyle was given a night off on Tuesday, giving Dylan McIlrath a chance to play for the first time since hurting a knee in early March.