Manchester United dealt blow as bid for main target is rejected
Ole Gunnar Solskjaer is keen to bolster his team's backline after the Red Devils shipped 54 goals in the Premier League this season.
The Norwegian has made the Napoli centre-back his leading target, but United's initial offer has been knocked back by the Serie A side.
The Partenopei are determined to keep hold of the Senegal international, who is contracted at the Stadio San Paolo until 2023.
Koulibaly will be available for £130m when a buy-out clause in his deal kicks in next summer.
And United will have to up their offer if they want to land the former Genk stopper ahead of the 2019/20 campaign.