"I could see his potential, [but] I never thought he'd be an international player," said Adams, who handed Maguire his debut back in 2011 when he was just a gangly 17-year-old from the Blades academy.
Not only did the Leicester City centre-back become an international player, he earned immense praise from the merciless English press for his performances at the World Cup.
Alas for Mourinho, Maguire has remained at the King Power and rather ironically will line up against United in the opening Premier League fixture on Friday night.
So, how will the events of recent weeks affect Maguire? Are footballers adverse to such matters or will a lingering anxiety simmer in Maguire's consciousness?
The former was certainly more prevalent during my short time as a professional footballer with Hamilton Academical.
From an outsider's perspective football might play out like a movie. But all the hoopla surrounding speculation and transfers is fought in the boardroom between directors and agents. From what I observed, players only concentrated on one thing every day: fighting to keep their job. Every week there would be a loan move agreed or a trialist having their dreams either realised or shattered. In or out. Coming or going. For the clubs, football is a business. For the players, football is employment.
And Maguire, like any other professional, will do his job to the best of his abilities tonight. Eyes will be focused on whether he has been affected by the collapse of his dream move. All Maguire will be focused on is containing either Romelu Lukaku or Marcus Rashford.
Both pose different challenges for Maguire. Lukaku is the ideal match-up given his propensity for playing with his back to goal, however, his class means he would trouble any defender in the world. Rashford on the other hand can expose Maguire's biggest weakness: speed.
Leicester will likely sit deep at Old Trafford, but there will be times when they push up the ground. And from personal experience, it is unnerving when you're a slow centre-back and you turn around and see your goalkeeper 50 yards away.
Being 18, I was naive and arrogant enough to think I could trail Hasselbaink wherever he went. Working off instinct, I was trying to preempt a pass so I could intercept the ball, lead a breakaway and be showered with praise after we scored down the other end. In reality, I had forgotten that my agility was reminiscent of a Mack truck trying to throw a sudden U-turn and the Dutchman was licking his lips. After 90 minutes, two goals had gone into my own net and I walked away with tired legs and sore ears from the rollocking I received from the gaffer.
Comparing myself to Harry Maguire is like comparing apples to oranges, but the foundation of the lesson I learnt that day in Paisley is something Maguire is continuing to learn on a much grander scale. For many scouts, pace is a prerequisite for a professional footballer in today's game. But if you make your strengths formidable, they can mask your weaknesses. Maguire needs only to look at Diego Godin or Per Mertesacker to know that you can become a world-class stopper if your brain works quicker than your feet.
He isn't there yet. And given his contentment after the failure of his dream move, perhaps he knows it as well. Observing, accepting and then taking the necessary steps to overcome a challenge is something Maguire has done ever since Micky Adams handed him his debut. He has only played two full seasons in the Premier League and three seasons ago he ran out for Hull City's U23 squad. Now, he is attempting to establish himself in an elite class.
That challenge starts tonight at Old Trafford, where Maguire will try to dominate the same players he one day hopes to call his teammates.