Just 18 years old, Mainoo was a symbol that, perhaps, all was not lost for a fanbase who had watched the club Sir Alex Ferguson built wither away before their eyes over a mostly miserable decade. A cup win over an admittedly knackered Manchester City was never going to be a fix-all result, but with fresh impetus in the boardroom and the promise of a cultural overhaul, there was justification for cautious optimism.
Plenty have referred to the 'iconic' (as United themselves put it) photograph of Mainoo, Alejandro Garnacho and Rasmus Hojlund sitting together the Old Trafford advertising boards during a comprehensive win over West Ham in February 2024 as another false dawn. Fast forward a year-and-a-half, Hojlund is off Napoli, Garnacho is being sold to Chelsea for a relatively paltry £40, and Mainoo has requested he be allowed to leave on loan.
None of this will come as a surprise to anyone who has even loosely followed United's fortunes in recent years. Good things simply do not seem to last. Players and managers arrive with bubbling enthusiasm and burgeoning reputations, only to find themselves worn down and weary by the strain of it all.
Hojlund less so, as the forward has always looked burdened beyond his ability in a United shirt, but that Garnacho and Mainoo have been deemed expendable suggests that, yet again, something has gone badly wrong. It was only a year ago that both were labelled indispensable, the catalysts for an invigorated future.
How did it come to this?
The most basic explanation is that manager Ruben Amorim does not seem to fancy either player. His issues with Garnacho, a significant degree of it centred on attitude, have been clear-cut, and the 21-year-old Argentine hasn't done much to help himself at times.
Mainoo feels like the real tragedy, however. A United fan since boyhood, the midfielder has lived and breathed the club almost his entire life. As well as the cup final masterclass, there was the breath-taking stoppage time winner away at Wolverhampton, and a glorious finish at home to Liverpool, all of which cemented his place as a supporters' favourite.
Paul Scholes compared him to Zinedine Zidane. He started for England in the Euro 2024 final. At just 20, he stills wrangles with the inconsistency of youth, but his talent and potential seems unquestionable. And yet Mainoo feels that United is currently not the best place for him to continue his development.
Amorim has declared publicly that he will compete with captain Bruno Fernandes, who is rarely even subbed off never mind put on the bench, for a place in the team. With no European football to contend with, and no League Cup following Tuesday's humiliating defeat to League Two side Grimsby Town, it seems Mainoo foresees too few opportunities considering United will play once a week for most of the campaign.
In such circumstances, there is a school of thought that such a young talent should look to knuckle down, but United look as poor as they ever have with Amorim in charge. The manager appears to prefer the ageing Casemiro and the limited Manuel Ugarte alongside Fernandes in midfield.
As United sought a winner against Fulham a week ago, Amorim brought on two defenders in the closing stages and left Mainoo sitting alone with his thoughts. It's been reported that the manager has reservations about his mobility, but it would be difficult for him to argue that anyone else in that midfield is performing at undroppable levels.
However you view the situation, it feels rather sad that it's come to this at all.
United have reportedly rejected Mainoo's request for a loan. It's been briefed that he currently has no desire to leave the club permanently, which arguably suggests a belief that his contract will outlast Amorim's tenure.
He might be right.
Amorim is an earnest, likeable figure who achieved brilliantly sustained success at Sporting CP. And yet there is a sense that his dogmatic approach has made him his own worst enemy. Across all competitions, he has won only 16 of his 45 matches in charge. His current record is among the worst in Premier League history.
Amorim has been criticised for tactical rigidity and his insistence on sticking to the 3-4-3 system he refined at Sporting. Formations are a bit of a red herring in modern football, so fluid and changeable are they in-game. For example, a team's shape in build-up can be completely different when they don't have the ball. But for Amorim, the issue is less the on-paper formation and more that even after several months in charge and a full pre-season for his players, they still look lost within it.
Connections are more important than starting shape, and United still don't possess a fluency in attack. Out-of-possession, they are still too easily exploited. The logic in deploying Fernandes in a deeper midfield role seems questionable, and can you really get the best out of Amad Diallo as a wing-back? Amorim's use of dual number 10s leaves space for only a midfield two, which goes against the grain when you look at how elite sides tend to set up these days.
But the biggest challenge facing the United manager is simply getting some wins on the board. Pre-season optimism has dissipated almost entirely by late August, but most of the above questions will subside if United can start stringing wins together. Rebuilds take time, especially at a club as rotten as this, but you only get buy-in if the inevitable setbacks are offset by genuine signs of progress.
Amorim has had 45 games to sketch out what the end result might look like, but the picture still looks a mess. It's much easier to make the case that United have gone even further backwards.
Post-Grimsby, he looked and sounded like a defeated man. Having seen such managerial churn post-Ferguson, there is little appetite for another change in the dugout, but that doesn't mean Amorim isn't running out of time.