The centre-back was sent off after being targeted with monkey chants when Napoli were beaten 1-0 at Inter Milan on Boxing Day.
Jose Mourinho had lined Koulibaly both in the summer and in January as he laid down plans to strengthen his defence.
The request was made to Ed Woodward to bring him in but it fell on deaf ears – but McGarry thinks that’s only because chiefs didn’t trust Mourinho’s calls.
Ian McGarry told the Transfer Window podcast: “Ed Woodward, at the point when Jose Mourinho was pushing for Koulibaly to be bought in the January window, I think both Woodward and the Glazers had lost faith in Mourinho’s abilities, maybe even before.
“Remember that he bought Victor Lindelof and Eric Bailly, neither of who have covered themselves in glory, and their argument was, ‘Why should we spend €100m on your next guess to play centre back?’
“I think Koulibaly is well above both Lindelof and Bailly so that doesn’t really hold water that argument.
“Koulibaly, I think, could fit in to any team in European football, any big team.
“He has everything in terms of his reading of the game is very, very good.
“His positioning both on and off the ball is intelligent and he’s hard. He’s one of those proper hard, hard men defenders, you know?
“He takes no prisoners, I’m not saying he’s dirty in any way, but you know you’ve been in a game when you play against him.
“There’s no doubt that he will be of interest again to many clubs around Europe.”
Napoli’s chairman Aurelio De Laurentis claimed to have rejected a £95million bid from United in the summer. However, club sources insist interest never went past a valuation inquiry.