Manchester United are desperate to sign a central defender in the January transfer window.

And the main target of manager Jose Mourinho is Kalidou Koulibaly, according to journalist Duncan Castles.

The Napoli defender is rated as among the best in his position.

But even though the Italian side are out of the Champions League, it would take a record sum for a defender to convince them to sell midseason.

And it is unclear if United would back Mourinho with that kind of money, given his own future at the club is in the balance.