The Swede faces a long recovery period after suffering the injury in victory at Tottenham last Saturday.
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.