OTTAWA -- Erik Karlsson's season is over.

The Ottawa Senators defenceman had surgery Thursday morning after his Achilles tendon was cut Wednesday during a game against the Pittsburgh Penguins.

Senators general manager Bryan Murray says Karlsson's surgery went well and the defending Norris Trophy winner's tendon was 70 per cent cut.

Karlsson's recovery time is expected to be at least three-to-four months.

Karlsson was injured while being checked by Pittsburgh's Matt Cooke, who was not disciplined by the league.

The loss of Karlsson is a huge blow to Ottawa (7-5-2) which is already minus star forward Jason Spezza, who is out indefinitely after having back surgery for a herniated disc.