Manchester City identified Koulibaly as their top defensive target and held talks with Napoli via an intermediary in a bid to reach an agreement.
They could not do so and have since switched their attention to other targets with, according to Italian broadcaster Sportitalia, talks over Koulibaly now coming to a firm stop.
Manchester City returning for the Senegal international has not been ruled out, but time is running out and Napoli are now planning for the eventuality that Koulibaly is not sold.
They remain keen on Arsenal defender Sokratis and had been planning for him to replace Koulibaly; Sokratis has been trying to convince Arsenal to terminate his contract.
Napoli could look to the Greek to come in for either Nikola Maksimovic or Sebastiano Luperto.
Coach Gennaro Gattuso is a fan of Sokratis’ abilities and would like him adding to the squad.
If he does not arrive in the ongoing transfer window, he could land next summer, when his contract at Arsenal will have run out.
