That is the verdict of ESPN FC pundit Paul Mariner.
United are back in business after appointing Ole Gunnar Solksjaer as interim manager, winning 10 of his 11 games in charge.
But they are looking to reassert themselves as Premier League contenders rather than participants, with a battle for the top four being their main aim this season.
The Red Devils will have to spend big in the summer and one position they will strengthen is at centre-back.
And there is only one man on United’s shopping list – Koulibaly.
The Senegal international has established himself as one of the best defenders in the world but Napoli are playing hardball.
However, with United’s spending money they will be able to break the bank if they really want him.
And former Arsenal ace Mariner reckons that this will be the case – but the 27-year-old could reject a transfer.
“Do Manchester United need him? 100 per cent they do,” Mariner said.
“Will they get him? Because this guy, there’s not many around [like him].
“Everybody talks about him and everybody wants to make a bid for him.
“Will Napoli crumble eventually? Of course they’re going to because of these massive clubs coming in.
“Koulibaly to Man United? He would be fantastic for Manchester United. Fantastic. And he would help them.
“But this guy may want to go somewhere where can win immediately and I’m not sure he can win immediately at Man Utd. I’m talking about the big stuff.
“I’m not talking about getting into the Champions League, I’m talking about winning big stuff.
“I love the guy, he’s fantastic.”