Callahan, who scored the winning goal against the Flyers, was injured early in the third period during a scuffle with Philadelphia forward Max Talbot.
After undergoing an MRI on Wednesday, Callahan was diagnosed with a subluxation of his left shoulder. The forward was dragged down by the arm by Talbot and immediately skated off. His left arm dangled as Callahan went down the tunnel to the locker room.
In the second period, Callahan scored to give the Rangers a 2-0 lead. It was his 200th point in the NHL.
If the injury timetable is correct, Callahan would miss six games over the next two weeks - starting with a home matchup against Atlantic Division rival Pittsburgh on Thursday night.
With the NHL playing a condensed, 48-game regular season because of the long lockout, Callahan will likely miss more games than he would have in a standard season. His absence will be felt in many areas, most notably on the defensive end and in penalty-killing.
"I don't think you fill his role," Rangers coach John Tortorella said Wednesday. "I felt that in the third period [Tuesday] night when we were protecting the lead, that's a very big part of his game.
"Every team goes through the injuries, and just playing that much better in the situations that he's been put in through his career, it needs to be done as a group. You lose a top guy like that, it shuffles the lineup, and it gives other guys chances that they probably wouldn't get if he was playing. I feel comfortable in us joining together as a team and trying to get it solved until we get him back."
The Rangers were already concerned about their depth at forward and came close to signing veteran Jason Arnott, but he failed a physical over the weekend. New York's offense has been fueled, so far, by the top line of Rick Nash, Brad Richards and Marian Gaborik.
Finding secondary scoring has been a concern for Tortorella, but the Rangers (3-3) have won two straight and three of four.
Now they will have to go forward without Callahan, at least for a little while.
"He's been a key part in everything we do here," linemate Derek Stepan said.
Callahan was injured 4:42 into the third period Tuesday when he and Talbot got into an altercation at the side of the New York net after play was stopped.
The players were engaged, and Talbot dragged Callahan down to the ice by his arm. Both players were given roughing penalties.
Philadelphia scored a power-play goal moments later following another Rangers penalty, but the Flyers couldn't take advantage of Callahan's absence on two other power plays later in the period.
"Cally does everything so well -- the little plays, especially blocking shots," Rangers defenseman Michael Del Zotto said Tuesday night. "The guys did a good job of gathering ourselves after he went off with that injury."