The Swedish defenseman could face a race against time to feature in next year's 2014 Winter Olympics after the injury during Ottawa's loss to Pittsburg.
NHL: Karlsson out indefinitely after achilles tendon cut in clash with Matt Cooke
Erik Karlsson is out indefinitely and will need surgery after suffering a cut to his left Achilles tendon in the Ottawa Senators game Wednesday.
The reigning Norris Trophy holder as last season's top defenceman, was injured in the second period of the Senators 4-2 loss to the Pittsburgh Penguins.
The 22-year-old Karlsson was killing a penalty when Matt Cooke, the Penguins' forward, came up behind him along the boards. Cooke's skate appeared to slice Karlsson near the left ankle.
In obvious pain, Karlsson was unable to put pressure on the leg and had to be helped off the ice. The Senators said he would need surgery to repair a torn tendon and could be gone for the season.
Paul MacLean, the Ottawa head coach, said afterwards that the injury was "unfortunate".
"Injuries are a part of the game," he added.
"We're going to have it re-evaluated when we get back to Ottawa."
There was no penalty called against Cooke - one of the most suspended players of his time - who insisted the injury was accidental.
"I feel terrible," he said.
"I'm not trying to do that, obviously. It's happened a few times over the past few years. It's scary. I was trying to hit him. Not like that."
But MacLean said: "We all know who was involved in it."
Karlsson led all Swedish defencemen in scoring in both the 2012 and 2010 World Championships. Sweden is hoping he will be healthy in time to anchor their defence at the 2014 Winter Olympics next February in Sochi, Russia.
Elsewhere, Alexander Steen scored the winner in overtime as the St Louis Blues beat the Detroit Red Wings 4-3 to halt a five-game slide and Michael Cammalleri scored a hat-trick in the Calgary Flames' 7-4 win over the Dallas Stars.
Follow us @SprtNationalUAE