The Red Devils are desperate to strengthen their defence this summer after enduring another troublesome season at the back.
United have been heavily linked with a move for Napoli defender Koulibaly in recent months as speculation remains over his future in Italy.
The centre-back won’t come cheap this summer, however.
And BBC reporter David Ornstein admits Koulibaly’s price tag is likely to mean a transfer will not materialise.
“One of their priorities is at centre-half as we know,” he told BBC 5 Live.
“Kalidou Koulibaly from Napoli. I’m told they love him.
“And the only problem with that is that he is extremely expensive. Napoli don’t need to sell.
“I think in 2021 he has a buy-out clause of €150m (£130m) and he has a few years left on his contract.
“So either they’ll be looking for a younger talent who can emerge to make that position his own or somebody proven.”
Should United try to buy Koulibaly this summer, a huge chunk of their transfer budget will have been blown.
And this would surely mean a move for Jadon Sancho would be out the window.
“The big one which not people have spoken about is Jadon Sancho,” Ornstein continued.
“Now Jadon Sancho is among the top targets at Manchester United I understand. And Manchester United would be his preferred option if he was to come back and return to the Premier League during this transfer window.
“I’m told that a couple of months this was looking like it would be nailed on.
“However, he [Sancho] is reluctant to give up Champions League football. He is reluctant to join an unstable club. He wants stability and he wants to win things.
“I’m told ‘never say never but as things stand it’s 50/50 on that one’.
“PSG, Juventus, Real Madrid and Barcelona have also shown firm interest in him. But there’s been no other interest from Premier League clubs.
“He won’t move for the sake of it, it has to be right for him and the fee this summer would be in excess of £100m.”