Manchester United have reportedly seen a €103m (£91.3m) bid rejected for Napoli centre-back Kalidou Koulibaly.

Defensive reinforcements are at the top of Jose Mourinho's priority list for the January transfer window, having been frustrated by the club's failure to strengthen in that area during the summer.

Koulibaly is understood to have emerged as one of United's preferred targets after club officials vetoed a move for Tottenham Hotspur defender Toby Alderweireld.

Italian outlet Corriere dello Sport reports that the Red Devils have now made an official bid for the 27-year-old, but saw it immediately rejected by Napoli president Aurelio De Laurentiis.

The deal would have seen Koulibaly become the most expensive defender in football history, smashing the £75m record currently held by Virgil van Dijk.

Senegal international Koulibaly joined Napoli for just £6.5m in 2014.