The Portuguese coach has made no secret of his desire to sign a new centre-back, having failed in attempts to sign the likes of Toby Alderweireld, Harry Maguire and even Diego Godin in the summer.
But now The Evening Standard report that United CEO Ed Woodward has promised Mourinho he will eclipse the £75million Liverpool spent on Virgil van Dijk if the right man becomes available.
Mourinho believes that man is Koulibaly, who has been brilliant for Napoli over the past few seasons, though the Italian side could hold out for as much as £100m.
That would make Koulibaly the most expensive defender of all time, and Woodward has made it clear that the funds are there to do a deal.
Mourinho is a big fan of the Senegalese centre-back, who impressed against Liverpool in the Champions League on Tuesday night, though he is not the only defender in his sights.
United have not given up on luring either Alderweireld or Maguire to Old Trafford, while Inters Milan Skriniar is also on their radar.
Mourinho is adamant that he wants defensive reinforcements as soon as possible, in January, but any deal for Alderweireld would have to wait until the summer when a £25m release clause kicks in.
Koulibaly is seen as being ready-made for the Premier League while Napolis failure to qualify for the last 16 of the Champions League means he could be registered for the knockout stages by United.
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