The former Porto defender, who moved to United in a €22m deal, was greeted by his new manager describing him as "the best fullback in Europe".
Of course, Mourinho was declaring Dalot the best "in his age group", but the claims still generated plenty of cynicism amongst local media and supporters of rival clubs.
However in Portugal, Mourinho is no Robinson Crusoe when it comes to his admiration for Dalot. Here's what has been said about the 19 year-old defender by some of Portuguese football's biggest identities in recent years:
SERGIO CONCEICAO (Porto coach): "He's a kid who has done an exceptional job in the B team. I take this opportunity to send a hug to (coach Antonio) Folha, because the kid's emergence and this work we see here today has to do with Folha's work in team B.
"Dalot has had a very good start. But success has nothing to do with one or another good performance, but rather producing continuity and more and more asserting himself with FC Porto, and he has the quality to do so.
"He is a great player."
JORGE NUNO PINTO DA COSTA (Porto president): "Dalot was the prize of the year. I believe he's for our future and to compensate for the present.
"He is already a reality of our football, he has been 10 years inside the club and I hope that for many more he will be here present to receive many trophies."
PEDRO MIGUEL (Porto youth coach): "He is very strong physically, it is natural that they can compare him to Cristiano Ronaldo, because he can also play on the wing because he is very fast and aggressive. He can produce goals and assists, there's nothing lacking in him as a winger.
"It's not a strange position for him, he's played there a lot of times, of course he feels better as a right-back, but he still has time to adapt, and I'm sure he could.
"He has physical, tactical and even technical qualities to play in attack, no doubt."
MIGUEL LAYUN (Porto player): "With his potential, character and personality, he can go as far as he wants. I would love to see him as one of the best in the world in a few years' time."
LUIS TRAVESSA (junior coach at Fintas): "Diogo is fortunate to have a fabulous family, who collaborated with the Fintas School, and was always at his back. It is very important, for any young person, at the end of the training to have the affection of the mother, the sister, the family.
"He was very focused, he wanted to know the reason for things, he wanted to repeat to improve. That set him apart. It was applied in training, at a time when most young people are young in the game.
"Because of this training interest, I think Diogo will be a great coach, too. I have no doubt about it."
LUIS BOA MORTE (ex-Portugal, Arsenal player): "Diogo Dalot will have the advantage of going to work with a Portuguese coach. I had all the difficulties and a few more. It was different, now they make everything perfect for a player to succeed. In my time, it was rare for English clubs to hire young foreign players, and Portuguese even less.
"The current generation of players is different, they are better prepared and, as such, the adaptation is much easier.
"Who is the young man who would not like to play for Manchester United? God wants everything to go well for him."