Faouzi Ghoulam's spell on the sidelines could well be extended with Napoli announcing they suspect the left-back has fractured his right patella.
Algeria international Ghoulam has not featured since tearing the anterior cruciate ligament in the same knee in the 4-2 Champions League group stage defeat to Manchester City on November 1.
The 27-year-old was cleared to resume first-team training last month but he has suffered a setback in his recovery.
"During [Friday's] training session, the Napoli defender injured his right knee," a statement on Napoli's website read.
"Ghoulam will be visited by Professor [Pierpaolo] Mariani at Villa Stuart. A fracture of the patella is suspected for him."
Napoli possess a one-point lead over Juventus at the top of Serie A and face Lazio at the Stadio San Paolo on Saturday.