Milik suffered the damage while on international duty during Poland’s 3-2 win over Denmark on Saturday.
The 22-year-old has been in excellent form for Napoli since moving to the Serie A side over the summer, scoring four goals in the Italian top flight already, but now he is set to miss several months of action.
The club have confirmed, via their official website, that Milik has indeed ruptured his ACL in his left knee and the striker will undergo surgery to repair the problem on Monday morning.
The statement from Napoli read: “Arkadiusz Milik today underwent an MRI at Villa Stuart, performed by Dr. Mariani, after the injury suffered while playing for Poland.
“The tests confirmed the diagnosis of a total rupture of the anterior cruciate ligament in his left knee.
“The Blue striker will undergo surgery tomorrow morning at Villa Stuart.”
While the club has put no time frame on Milik’s recovery he is likely to be out well in 2017 and there is a possibility, depending on the extent of the set-back, that he could miss the rest of the season.