Napoli have confirmed full-back Faouzi Ghoulam tore his anterior cruciate ligament in Wednesday's defeat to Manchester City.
The Algeria international was taken off in the first half of the 4-2 Champions League loss at the San Paolo and was replaced by Christian Maggio.
Ghoulam went for tests on Thursday, which identified a tear in his right ACL. He will undergo an operation to repair the damage on Friday.
Napoli did not put a timescale on his recovery but initial reports suggest he will be sidelined for approximately five months.
Head coach Maurizio Sarri felt his side were unbalanced by Ghoulam's injury in Wednesday's game, with City cancelling out Lorenzo Insigne's opening goal just three minutes after the defender went off.
"In my view, we lost momentum with Ghoulam's injury as it took a while to re-organise and it becomes difficult to win back possession when they press you that high up the field," he said to Mediaset Premium.