The 18-year-old wonderkid is finalising the paperwork that will allow him to join the Red Devils in January
Diallo, 18, agreed to a move to Old Trafford worth up to £37 million ($48m) over the summer.
But the clubs agreed to keep the youngster in Italian football until he was granted a work permit in the United Kingdom.
The Ivorian has since received an Italian passport and, as United boss Ole Gunnar Solskjaer revealed, has cleared the last legal obstacles preventing him becoming a United player.
“We have had all the paperwork, I can’t see any obstacles. He’s coming,” Solskjaer told reporters following the Reds’ 2-1 victory over Aston Villa on Friday.
“I hope very soon he will be with us. I’m very excited about bringing him in. I think he’s shown a few times his qualities.
“Of course he has to learn to play in England, that’s a different level of football. We’ll give him time to develop and to express himself.
Apart from that, I’m not really thinking too much on what we’re going to do with players. Of course, we’re always working long-term. That’s the big thing. We have targets we need long-term for the club.”
On Sunday Diallo posted from the plane purportedly flying him to the Italian capital in order to complete the final paperwork ahead of confirmation of the transfer.
Reports suggest that the teenager will then fly to Manchester in the coming days to join up with his new team-mates.
Atalanta were keen to hold on to their young gem until at least the end of the season, but coach Gian Piero Gasperini conceded that such a prospect now looked impossible.
It’s difficult because Manchester United want him to join the club immediately in January,” Gasperini told Sky Sport Italia.
“Amad has received his passport.”
Diallo has made two appearances for Atalanta in the 2020-21 season and overall has played five senior competitive matches, with his first and only goal to date coming against Udinese in October 2019.