Napoli manager Carlo Ancelotti has described Kalidou Koulibaly as the "best centre-back in the world" and insists that he is not for sale.
The Senegal international has been regularly linked with a move away from the Stadio San Paolo in recent times, earning particular interest from Manchester United.
With the Red Devils still on the hunt for a new central defender, reports indicate that they may move for Koulibaly in the summer.
Ancelotti has categorically ruled out an exit for his prized asset, however, telling reporters: "Koulibaly is the best centre-back in the world, and he has more room for improvement.
"He is worth €150m and he is non-transferable. Napoli do not need to sell, the president does not want to sell and the question does not arise."
Koulibaly has been used 43 times in all competitions this season, including in both legs of Napoli's Europa League quarter-final defeat to Arsenal.