Manchester United have failed with a new offer for Napoli defender Kalidou Koulibaly.

Corriere dello Sport says Napoli have turned down a £91m offer for Koulibaly from Manchester United.

The bid was 'immediately' rejected by club president Aurelio De Laurentiis.

United boss Jose Mourinho's frustration at his defence has been well documented, and he has lamented the fact he has been unable to bolster his back line with more talent.

The current world-record fee for a defender is the £75million Liverpool paid to Southampton for Virgin van Dijk, however United have been looking to smash that with this bid for Koulibaly.

However, with the apparent quick rejection from Napoli, it appears United will have to fork over even more cash for the Senegalese player.