That announcement came from general manager Bryan Murray on Thursday afternoon when he met with the assembled media in Ottawa.
Karlsson suffered a lacerated Achilles tendon Wednesday night when Pittsburgh Penguins forward Matt Cooke cut him with his skate just above the boot of Karlsson's skate.
Murray announced that Karlsson had already underwent surgery on Thursday and is expected to miss 3-4 months while recovering. The general manager said 70 percent of the tendon had been sliced and that Karlsson was not wearing kevlar socks, which are worn by some players to protect the exposed area above the skate.
Murray also addressed the NHL's decision not to pursue supplemental discipline on Cooke. He insinuated on Wednesday that he felt Cooke intentionally injured Karlsson and re-elaborated that belief on Thursday by bringing up Cooke's checkered past.
Despite the loss of one of the team's best players, Murray stated he still believed the Senators would make the postseason and that he wasn't going to trade a top-prospect for a short-term fix.