Karlsson, the reigning Norris Trophy winner and the top-scoring blueliner in the NHL this season, was injured Wednesday during the Senators' loss to Pittsburgh when the Penguins' Matt Cooke collided with him along the boards.
Cooke's skate blade came off the ice during the hit and inadvertently came down on Karlsson's exposed Achilles.
Murray said Thursday the 22-year-old defenseman suffered a 70- percent laceration of the tendon and underwent surgery at The Ottawa Hospital to repair it, the Canadian Broadcasting Corp. reported.
The GM said Karlsson will be out 3-to-4 months, effectively ending his season, and added he is not seeking discipline for the oft-suspended Cooke.
The injury was the latest in a series suffered by the Senators, who have also lost high-scoring forward Jason Spezza for the season to back surgery and defenseman Jared Cowen to hip surgery.