Koulibably has emerged as United's prime defensive target, but Napoli are not letting him leave on the cheap.
Former Red Devils star May told 888sport: "If you look at United over the last God knows how many years they've always gone out and broke transfer records if need be.
"They did with Rio [Ferdinand], [Angel] Di Maria, [Paul] Pogba, [Romelu] Lukaku. They allegedly pay [Alexis] Sanchez half a million a week.
"If you want the best players then you have to pay top money for them. City do it.
"Liverpool did it with Van Dijk and just look at the transformation.
"They spent £70 million on a goalkeeper, which is a hell of a lot of money but what that gets you back is guaranteed Champions League football and the possibility of winning the Premier League.
"The amount of money United earn then to spend £100m on a defender….so what?
"If it guarantees you a much better defence then go ahead and spend it.
"If Koulibaly becomes available – and there are rumours that he is interested with a fee of £90m – then I'm sure Ole would say he'd be a great signing for Man Utd."