The 27-year-old has attracted the attention of Europe’s biggest clubs after a series of stellar seasons with Napoli.
Koulibaly arrived in Italy for just £6.98million from Genk in 2014 and his stock has now risen to over ten times of that amount.
A centre-back will be high on United’s priority list this summer after they failed to land one in the market before Jose Mourinho’s departure.
The Portuguese saw moves for Milan Skriniar, Harry Maguire, Toby Alderweireld, Diego Godin and Jerome Boateng all fall short.
Since the arrival of Ole Gunnar Solskjaer, Victor Lindelof has been one of the standout performers in his Manchester United side which has eased the immediate need for a central defender.
While Chris Smalling has also impressed in recent weeks, a long-term partner for the Swedish international will still be on the agenda.
According to Calciomercato, United are preparing a ‘mega-offer’ to try and land Koulibaly this summer.
The Senegal international signed a two-year contract extension with Carlo Ancelotti’s side in December, which makes their chances of landing him increasingly difficult.
Upon the deal, Napoli president Aurelio De Laurentiis made it clear that the star will not be leaving the Stadio San Paolo.
“Koulibaly, he has renewed with us for another five seasons,” he said.
“He is the best defender in the world probably.
“For him I have refused €90 million from three clubs because I do not sell.”
Although there is yet to be an official announcement, barring a severe turn of events Ole Gunnar Solskjaer will become the next permanent manager of United.
When asked whether he has given advice of player renewals and departures, Solskjaer said: “Yes I have. We’ve been discussing the future of the club.
“We did that before I had won two or three games and all this talk about if I should stay or not.
“My contract is up in the summer but I still, as I’ve always done for Man United, give my 100 per cent and my best.
“You have to plan. Even if I’m not here, I still give my view on who I would like to see as a supporter.”