Ever since reaching an agreement with the Serie A outfit on October 5 - the final day of the summer transfer window - preparations have been under way for the 18-year-old's Old Trafford arrival.

On Thursday, Diallo completed a medical, work permit and personal terms, signing a deal until 2025 at United with the option of a further year.

The initial fee for the winger is understood to be 21 million euros ($A33 million) and add-ons could see it reach 41 million euros ($A65 million), with Diallo now just awaiting visa clearance before travelling to Manchester.

"As a club, we have followed Amad for a number of years," United boss Solskjaer said. "And, having watched him myself, I believe he is one of the most exciting young prospects in the game.

"Manchester United has such a proud history of developing young players and everything is in place to enable Amad to reach his potential here.

"It will take time for him to adapt, but his speed, vision and fantastic dribbling ability will stand him in good stead to make the transition.

"He is a player with all of the raw attributes that are needed to be an important player for Manchester United in the years to come."

Considered one of the most exciting prospects in Italy, Diallo has been on United's radar since 2016 and consistently received the highest ranking when scouting that age range.

"After waiting since the summer, it is finally a dream come true to complete my move to Manchester United," said Diallo, whose shirt number has yet to be revealed.

Australian Associated Press