The injury occured late in the second period of their 4-2 loss in Pittsburgh as Karlsson and Penguins forward Matt Cooke went for a loose puck along the boards and Cooke's skate came down on the back of his leg.
Karlsson instantly dropped to the ice and had to be helped off to the Senators locker room as he couldn't put any weight on his leg.
It's a huge blow to the Senators as Karlsson is not only one of the best players -- perhaps the best player -- on the team, but he's also off to a fantastic start this season and is perhaps even better than he was last season when he won the Norris Trophy.
"Well, he's playing 30 minutes a game, he's the Norris Trophy winner, and he's arguably the best player in the league," said Senators coach Paul MacLean. "It's obviously going to be a big loss, but like with every injury it's somebody elses opportunity and we'll have to see who it's going to be."
Because the play involved Cooke, one of the NHL's most well-known agitators and a player with a lengthy track record of questionable play throughout his career, the discussion instantly turned to whether or not it was intentional.
Cooke said he didn't even know what happened on the play and only heard Karlsson screaming.
"It's a complete accident," said Cooke. "Obviously, I feel terrible about it. I'm not trying to do that, obviously. It's happened a few times over the past couple of years. It's scary."
Nobody from Ottawa came out and openly accused Cooke of a dirty play, but the mood wasn't exactly a good one in the Senators locker room, either.
General manager Bryan Murray appeared to be livid after the game and quickly told the assembled media "It's Matt Cooke, watch the replay" before walking out of the locker room.
"I saw it quick so I don't know if it's intent to injure, but I don't know why you would hit somebody like that in that situation," said Senators captain Daniel Alfredsson. "I can't tell if it's intent or not. Obviously, he catches Erik very unfortunately right where his skate ends and we don't have any padding."
Said MacLean, "It's unfortunate it happens on a nothing play. We all know who is involved with it and that's just the way it is. Injuries are a part of the game and we just have to move forward."
When asked if he thought it was intentional, MacLean simply said "that's not up for me to decide."
The Senators are already dealing with some injury concerns as they're without center Jason Spezza. Winger Milan Michalek was also a late scratch on Wednesday night after suffering a lower body injury during warmups.