After being given the nod in previous friendlies, the 18-year-old was chosen ahead of Scott McTominay and Christian Eriksen to start against Real Madrid on the tour of the US. It was a night to remember for the teenager, but for the wrong reasons.
His game ended after two minutes when he was helped off the NRG Stadium pitch, unable to put weight on his ankle following a freak collision which saw Rodrygo fall onto his leg from a separate challenge.
Mainoo was not expecting this to be his second biggest challenge in senior football. His first came back in January when he made his Manchester United debut from the start in the League Cup quarter-final clash with Charlton.
He lasted an hour after giving Erik ten Hag no reason to take him off but for tiredness amid a scintillating shift. Mainoo was later rewarded with a Premier League debut against Leicester and an FA Cup bow against Reading.
The midfielder suffered a small injury towards the end of the campaign which prevented him from getting more minutes but he was fit in time for pre-season. He has been unable to take part in the first part of the regular campaign due to his ankle frustration.
If Mainoo had been included, he would have been used centrally, although he made his first two senior appearances as an attacking midfielder. He played the position nine times in the academy and walked away with three goals and an assist to his name, so he can do a job there.
With Sofyan Amrabat arriving on deadline day, Mainoo has more competition for the central spots – but more opportunities could open up further up the field. Bruno Fernandes has got the supporting striker role on lockdown and no-one is taking his place but for injury or illness.
On occasion, the United captain has been asked to play on the right. For whatever reason he is not used as the attacking midfielder, Mainoo could be next in line, something that will be concreted should Donny van de Beek exit next week.
Galatasaray and Fenerbahce – his most likely would-be destination – are interested in the out-of-favour Dutchman, with the Turkish window open until September 15th. The possibility of him exiting narrows with each passing day but the presence on Mainoo may make Ten Hags decision a little easier.
To most onlookers, a Van de Beek departure seems to benefit all parties but Ten Hag persisted with his former Ajax player up until his season-ending injury in January. But, ineffective in his 10 outings prior to his setback, he is yet to play competitively since.
He scored a goal against Lyon in pre-season but was largely anonymous for the rest of the summer. If Ten Hag is reluctant to let Van de Beek go, however, then his overheads might push him in that direction.