Koulibaly enters the final year of his Napoli contract, and a summer exit might be imminent. Chelsea are the newest serious addition to the list of suitors.

Napoli Intend to Keep Koulibaly Despite Barcelona Interest

Napoli will have to reshape the roster next summer as multiple players have expiring contracts, and replacing Kalidou Koulibaly is not currently in their plans. The center-back has been linked to Barcelona recently and has been routinely on the radar of top clubs. However, the club believes he will continue to stick around.
Differently from Juventus and Inter, Napoli are not planning any major investments in January. They will at most add a left-back on loan.

Napoli Zigging While Other Serie A Heavyweights Are Zagging

Differently from Juventus and Inter, Napoli are not planning any major investments in January. They will at most add a left-back and do not envision a shift in their strategy next summer either. Aurelio De Laurentiis intends to keep the books in order and considers the recent deals of the competitors rather reckless.
Lazio has been linked to Dries Mertens and Marcelo, who are currently without a team, but their president Claudio Lotito threw cold water on the rumors.

Dries Mertens Eager to Convince Napoli to Keep Him Around

Dries Mertens would like to stay at Napoli past his expiring contract, and he made it clear. “I have had a fatal attraction for this city and its people since day one. I have been here for nine years, which is a quarter of my life. I will try to score a lot, so the president is obligated to re-up my contract."