Rafa Benitez has revealed that Juan Camilo Zuniga's knee injury remains a "mystery" that the club's medical team are yet to solve.
The Colombia international has been out of action since October and it is still not yet known when he will be able to make his return to first-team action.
"I have spoken with Camilo," Benitez told reporters on Saturday. "But his knee is a little bit of a mystery.
"His situation is that he still has a little bit of discomfort. He trains and then, after two or three days, he's suffering.
"We'll continue waiting on him. He's working hard and he wants to return in order to give the squad a hand.
"But it doesn't depend on him. It depends on the knee."
Napoli booked their place in the last 16 of the Europa League with a 3-1 victory over Swansea on Thursday night, while they reached the Coppa Italia final by disposing of Roma last month.
It has been suggested that their cup exploits have hindered their Scudetto hopes, with the Partenopei having fallen 15 points behind Serie A leaders Juventus after what was a bright start to the season.
However, Benitez insists that he has no problem dealing with a congested fixture list.
"Playing twice a week means there is less time to prepare for games, but I prefer to play games rather than watch them on television," the former Liverpool boss stated.
"Also, facing teams from different championships is important as it helps you develop."