Manchester City boss Pep Guardiola is ready to splash the cash in the summer to improve his team.
Now that City are back in the Champions League for next season, Guardiola knows he can start planning for the future.
And part of those plans includes strengthening the defence to mount another Premier League title charge.
He wants a strong, experienced partner for Aymeric Laporte.
Koulibaly is happy at Napoli, but if City offer a fee in the region of £75 million, his club may have no choice but to sell him.
Liverpool have also been linked with the Senegal international.