The 18-year-old Ivorian right-winger has been on the radar of United scouts since 2016, when he was spotted playing at Under-15 level, and the club have been tracking his progress since. Many in Serie A feel Diallo is one of the most exciting young prospects in Italian football.
It was an interesting move in October because the two teams worked out an agreement that Diallo could remain in Italy and play for Atalanta the first half of the season.
The main holdup was the fact that Diallo was waiting to receive an Italian passport, which would enable him to get a work permit to join United, and that came through this week.
With that hurdle complete, Diallo is set to fly to England in early January to complete his transfer to United, a €25m (£22.38m) plus add-ons deal.
Diallo has made five appearances for the Italian side, scoring once, but has mainly been on the bench this season.
He is not eligible for the FA Youth Cup and United’s Under-23 side does not have a game until 18 January but with the new rules regarding teams having extra subs it is possible Diallo could be named in United’s senior squad, especially as Odion Ighalo’s loan from Chinese Super League club Shanghai Shenhua comes to an end next month.