The Swede faces a long recovery period after suffering the injury in victory at Tottenham last Saturday.
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Isak won;t be seen again until the final stages of the season after surgery this week.
Posting to social media after the operation, Isak stated: "Gutted to be out for a while. Time to recover and support from the sideline.
"I will work hard to be back as soon as I possibly can. Thank you all reds and everyone else for the kind messages, it does not go unnoticed. #YNWA"
Isak has just two goals and one assist across 10 Premier League games this sesaon. He also scored once in the Carabao Cup.