With the Rangers clinging to a one-goal lead with 1:45 left Monday, Callahan blocked a booming slap shot by Bruins defenseman Zdeno Chara, but taking one for the team was costly as Callahan broke his right leg.
Rangers coach John Tortorella said that it is a "long-term injury," and Callahan will be sidelined indefinitely.
On Tuesday following practice, Chara said he felt horrible for a player he considers to show a rare mix of skill and heart on every shift.
"Callahan is a top-six guy, yet he still plays with so much heart and grit," Chara said. "You don't see many guys that throw their bodies around to block shots like that. He plays the game so hard. You have to respect a guy like that. I just hope that he's OK long term."
Callahan is fifth in the NHL among forwards with 77 blocks, and the Rangers are second in the league with 672.
Callahan missed 19 games this season because of a broken hand sustained while blocking a shot by Pittsburgh defenseman Kris Letang on Dec. 15.
Callahan is second on the team in points with a career-high 48 on 23 goals and 25 assists. His production will be sorely missed by a team on the cusp of qualifying for the playoffs.
The Rangers went into Tuesday's action in seventh place in the Eastern Conference, a point ahead of eighth-place Buffalo with two games to play.
"Don't get me wrong, there is nothing good that comes out of this, but it can galvanize the team even more," Tortorella said.
The coach said that the Rangers would dress defenseman Matt Gilroy as a winger because the move "gives us some flexibility with having a seventh 'D' in there."
Captain Chris Drury also is making strides to get back into action. He has been skating in recent days and will take part in full team practices. Drury has played in only 23 games this season because of finger and knee injuries. He has been out since undergoing knee surgery on Feb. 11.
He isn't rushing back because of Callahan's injury, and his return isn't close, but he realizes how difficult it will be to replace what Callahan brings.
"You can't really say enough good things about him as a player and as a person and as a leader in this organization," Drury said. "We're going to miss him, and miss him a lot. He plays in every situation and does all the little things, does all the big things. He's just been great. He's going to be a big loss for us.
"As he would want us to do, want the guys to do, is play hard without him, kind of move on, win some games, and put a smile on his face that way."
Drury knows the peril of shot-blocking. He broke a finger for a second time when he got in front of a shot in the home opener on Oct. 15.
Information from ESPNBoston.com's James Murphy and The Associated Press was used in this report.