The Portuguese boss was in charge at Old Trafford between 2016 and 2018 and was tasked with managing a crop of players who, by his own admission, lacked maturity.
In an interview, with Hristo Stoichkov, Mourinho called out four players in particular - Marcus Rashford, Anthony Martial, Luke Shaw and Jesse Lingard - for their lack of drive.
Speaking in November 2018, Mourinho said: "They are lacking maturity. When I say that, I mean maturity at a personal level. We are more men, we are more mature, we were more prepared to face life. We were less protected. In England, they call it 'spoilt'."
Mourinho added: "Today, we're talking about 'spoilt kids'. Our children are more 'spoilt' than we were.
"I think kids these days have a different social life than us, and players have around them an 'entourage', and I mean a personal entourage, the people that surround them, that protect them more, give them more affection, more excuses. They reach maturity more slowly.
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"For example, this kid, Luke Shaw, who is currently the national team's left-back, when I arrived here two years ago, he didn't know how to compete. He had great potential, yes, but he didn't know how to compete.
"When we talk about Luke Shaw, [Anthony] Martial, [Jesse] Lingard, [Marcus] Rashford, we're talking about kids with great potential but still lack a word I can't say but that you like a lot… they lack a bit of that. Character, personality, as they say in Spain as well, bad blood, that natural aggression that you had."
Amazingly, three of the four players are still with the Premier League giants, with only Lingard moving on from the club in the years that have followed.
Lingard left the club in 2022 before spending a season with Nottingham Forest, however the 31-year-old has been without a club since leaving the City Ground last summer.
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Luke Shaw is perhaps the exception in the quartet, with the defender going from strength to strength for both Man United and England as he continues to be a key man under Erik ten Hag more than five years later.
Martial's time at the club looks to be coming to an end as he approaches the final few months of his deal, while Marcus Rashford is enduring a torrid time.
Ally McCoist questions Marcus Rashford's Man United future that could be past the point of no return after boozy Belfast antics
An ill-judged night out in Belfast has seen the forward at the centre of a storm at Man United, with the player missing last weekend's FA Cup clash with Newport County.
Ten Hag has since stated that it's 'case closed' with regards to Rashford's conduct, with the striker now back in contention to feature for the Red Devils.
However, with Mourinho's questions about three Man United star's professionalism still looming large, fans will continue to assess just how right 'the Special One' was in his assessment.