Here, some of the big names of Italian football have their say on United's new right-back:
MAURIZIO ZAMPARINI - PALERMO PRESIDENT
"Darmian? He's a great player. I bought him many years ago from Milan, then we had to loan him out. I blame our whacky coaches.
"He's a great talent who would do well in any team.
"He would do well for any team."
GIAMPIERO VENTURA - TORINO COACH
"Darmian is a great guy who has worked for four years with seriousness with us. He has always done what we asked.
"Now he has reached what is the dream for every player: to play in the Champions League.
"The four years of hard work have been rewarded with the stage of Old Trafford and Manchester United.
"You cannot question him. He can now play in the Champions League, which Toro could not offer him."
DELIO ROSSI - EX-PALERMO COACH
"Matteo was a centre-back in the Primavera of Milan; then, when we took him in, I saw that he also had good speed and moved to play him on the right; he did well in the Europa League and having to rotate players, he also played several matches in Serie A.
"He improved, especially in the awareness of his own ability, like all young guys.
"He just needed someone to show confidence in him. He's a player who maybe didn't initially play well and was immediately rejected.
"Darmian has grown a lot in self-belief and confidence. With us, his value went from €5 million to €15 million."
FILIPPO GALLI - EX-AC MILAN YOUTH DIRECTOR
"Darmian needed a different path, away from us, but I remember an old chat with the CEO (Adriano) Galliani, where years ago he asked me about who I'd bet on from the academy.
"I said immediately Darmian. He was always intelligent and adept at reading defensive situations.
"Now he has improved a lot in his career."
ARIEDO BRAIDA - BARCELONA TECHNICAL DIRECTOR
"I do not need to scout Matteo.
"I followed him for almost his entire career with the AC Milan youth team. How do I see him now? He's good and has room for further improvement.
"Now he is an important player, has also reached the national team. He's a serious guy and humble, I congratulate him."
URBANO CAIRO - TORINO PRESIDENT
"Thanks Darmian, they were four wonderful years.
"Thank-you for the seriousness, commitment and professionalism demonstrated in these years. All of Torino FC hopes for Matteo the best satisfaction for this new, amazing experience."
BENIAMINO ABATE - EX-AC MILAN YOUTH COACH
"You could see he had potential. But to imagine he would take the career path he has, you'd need a crystal ball.
"He was the prototype of the perfect youth player. He really was a boy of ten out of ten. The success, he deserves it all. And I'm sure that he will not change one iota. The secret is this: he doesn't let things go to his head, he remains the same as always.
"He had great qualities at 15, 16 years of age. Already he had the right values: balance is the basis for a great career, so he can go far. He is the mirror of why education is fundamental for football.
"Yes, Matteo was shy, but determined. He, however, was also technically good and had important skills. A professional, even as a child."