Ottawa Senators defenseman Erik Karlsson's season is "effectively over" after having surgery Thursday morning to repair a 70-percent cut in his left Achilles tendon.

General manager Bryan Murray said that Karlsson will be out 3-to-4 months.

Karlsson suffered the injury late in the second period of Wednesday's game against the Pittsburgh Penguins. Karlsson and Pittsburgh's Matt Cooke met along the boards while chasing down a puck, and Cooke's left skate appeared to graze Karlsson near the left ankle.

The reigning Norris Trophy winner had to be helped off the ice as he was unable to put any pressure on his left leg.

Karlsson had a team-high six goals and four assists in 14 games this season.