(Reuters) - Ottawa suffered a double blow on Wednesday when they lost standout defenseman Erik Karlsson to an Achilles injury during their 4-2 defeat to the Pittsburgh Penguins.

Karlsson, the reigning Norris Trophy winner for the NHL's top defenseman, sustained a laceration late in the second period when he was pinned against the boards by Pittsburgh's Matt Cooke.

The Senators said Swede Karlsson would require surgery and was out indefinitely.

Pascal Dupuis put the Penguins ahead in the first period before Ottawa grabbed a 2-1 lead through Stephane Da Costa and Jim O'Brien midway through the second.

James Neal restored the Penguins' lead with his ninth and 10th of the season before Sidney Crosby made the game safe with a wrist shot in the third.

Crosby also picked up a pair of assists for the Penguins(9-5-0). Ottawa (7-5-2) lost for the fifth time in seven games.

(Writing by Jahmal Corner in Los Angeles)