Callahan was examined by Dr. Andrew Feldman and underwent an MRI on his left shoulder Wednesday.
The forward was injured during a fight in the third period of the Rangers' 2-1 win over the Flyers, leaving the ice favoring that shoulder. Teammate Marian Gaborik said Wednesday that Callahan's shoulder popped out during his fight with Flyers forward Maxime Talbot.
If the timetable proves accurate, Callahan would miss between four and seven games during that stretch. A return in 10 days might allow him to play against Tampa Bay on Feb. 10 at Madison Square Garden; missing two weeks could line up with a return at home against the Islanders on Feb. 14.
"He's a huge part of our team," Rangers forward Derek Stepan said. "But everyone is going to have to step up, take a little bit of his pie, and try to do some of the things he does. It's going to take six, seven, eight guys to fill the role that Cally does."
Callahan, in his seventh year with the Rangers, isn't one who pads the stat sheet, but he brings consistency. He plays on the team's second line and has two goals in the first six games, with his second score coming in Tuesday's win.
On special teams, he's equally as important. He's an integral piece of the team's penalty kill and has scored two of the team's three power-play goals this season.
He's tied for second in the league with 27 hits and is sixth among forwards with nine blocked shots.
The previous time Callahan missed an extended amount of action was in 2010-11, when he played just 60 games after breaking his hand and later fracturing his ankle. In the three other full seasons he's spent with the Rangers, he's played at least 76 games.
"Hopefully it's short and he gets back quick. Obviously the mood around here is we want him back as soon as possible," forward Brad Richards said. "Big part of every part of the game. Power play, penalty kill, 5-on-5s. That being said, we've got to push on. It's part of the game we play."
With Callahan out, the Rangers could use a boost from Stepan, who plays on the second line with Callahan. Although he's fifth on the team with four points, he has yet to score a goal. Excluding Callahan and the Rangers' top line of Gaborik, Rick Nash and Richards, the rest of the team has just six goals.
"Individually I'm still working on my game, I still got a lot to sharpen up," Stepan said. "I think now with Cally out it's going to be most important to really get it straightened out now."
Callahan's absence will force the Rangers to shift some of their lines. He played with Stepan and Carl Hagelin on the second line Tuesday, and he was replaced during Wednesday's practice with Taylor Pyatt.
The team did not have any AHL call-ups for Wednesday's practice, and defenseman Stu Bickel skated with the fourth line. Rangers coach John Tortorella does not divulge his lineups.
"You lose a top guy like that, it shuffles the lineup. Gives other guys chances they probably wouldn't get if he was playing," Tortorella said. "I feel comfortable in us joining together as a team and trying to get it solved until we get him back."
The Rangers also will be without forward Chris Kreider for Thursday's game against the Penguins as he recovers from a bone chip in his ankle. He had a light skate Wednesday but is not sure if he will keep skating daily. Kreider has missed the past three games, one as a healthy scratch, and is listed as day to day.