Mourinho currently manages Alli at Tottenham Hotspur and it turns out they could have crossed paths much sooner in their careers.
The Portuguese coach explained he was aware of Alli prior to joining Spurs thanks to United legend Ferguson.
In a clip from Amazon's highly-anticipated 'All Or Nothing' series, Mourinho tells Spurs chairman Daniel Levy: "I told already Dele very directly that he doesn't train well, he is not a good trainer.
"I am not saying a disaster but I am not saying Harry Kane, Harry Kane is a very good trainer.
"Sir Alex Ferguson gave me only one bit of advice in two and a half years: Buy Dele Alli. 'That guy, with that mentality, the way he plays, the aggression he has in mind. This guy is the Manchester United player. Buy Dele Alli'.
"And he [Ferguson] has an eye for a player. But he [Alli] is not a good trainer, we need to find motivation for the guy."
Alli joined Spurs from MK Dons in 2015 and quickly established himself as a first team regular.
The 24-year-old has seen his influence diminish in the last two seasons however and he made just 25 appearances this season.
He temporarily got back to his best in the winter, not long after Mourinho took over from Mauricio Pochettino.
In a clip from the documentary, the two-time Champions League winning coach is heard telling Alli: "I am going to be a pain in the ass for you. And you are lucky because when I am a pain in the ass it means I like you."
He goes on to add per The Sun: "I'm not expecting you to be the man of the match every game. I'm not expecting you to score goals every game.
"I just want to tell you I think you might regret..
"You should demand more from you, not me demanding more from you. Not me. Not me. Nobody. You.
"I think you should demand more from you."
Mourinho has been tasked with bringing silverware to the North London outfit and he believes he is up to the challenge.
"I feel the club has very good players and players with a dream to win with this club," the 57-year-old stated per TeamTalk. "It is a dream to do something special with the club. They almost did, but they didn't. The challenge for all of us is to make something special happen."
Featured Image Credit: PA Images
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