Arkadiusz Milik will undergo surgery on Monday on a knee injury suffered while playing for Poland.
The Napoli forward was forced off at half-time in his country's 3-2 win over Denmark, and was sent for a scan on Sunday.
The results of the scan showed Milik has ruptured the anterior cruciate ligament in his left knee, meaning a lengthy spell on the sidelines.
Since moving to Serie A from Ajax at the start of the current campaign, the 22-year-old has scored seven goals at club level in all competitions.
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