One day shy of exactly four months since he last appeared for the first time, he produced an outstanding performance at the base of United&aposs midfield. Chosen ahead of summer signing Sofyan Amrabat to marshal the engine room, the youngster left his calling card, leaving onlookers in awe.
Mainoo&aposs only previous Premier League experience had amounted to just 10 minutes, coming on as a substitute in United&aposs 3-0 win over Leicester City at Old Trafford back in February. Based on Sunday&aposs evidence, you would never have realised.
Bringing technicality and composure to United&aposs midfield, Mainoo played a key role in helping the Reds record their biggest Premier League away win under manager Erik ten Hag to date. Controlling proceedings in midfield with measured and precise passing, the Stockport-born ace looked far more assured and composed than an 18-year-old making his full league debut should have done.
Operating in a cauldron of noise, created on the back of Everton&aposs recent 10-point deduction, it would have been easy for Mainoo to have gone into hiding and get caught up in the occasion. Instead, he treated it as an opportunity to impress Ten Hag, who had a bird&aposs-eye view of the youngster&aposs first league start from high up in the main stand.
Mainoo&aposs performance was so polished and impressive that it is going to be increasingly difficult for Ten Hag to leave him out of his starting line-up when United face Galatasaray in their penultimate Champions League group game on Wednesday evening in Turkey.
The Dutchman is going to need every member of his squad at the peak of their powers with United currently bottom of Group A and facing elimination from the competition they fought so hard to get back into last season at the first hurdle. Failure to make it into the last-16 will be a catastrophic failure.
Galatasaray beat United at Old Trafford at the start of last month, which was a result, in Paul Scholes&apos eyes, that could be the one that they are left ruing the most if they fail to escape Group A&aposs clutches. It was a damaging defeat and United, just as they did against FC Copenhagen in their last European outing, led twice and still failed to get at least a point.
In Denmark, United had to perform against the backdrop of a passion-fuelled crowd, who tried to make it as intimidating as possible for them. Rasmus Hojlund&aposs early brace silenced them, before Marcus Rashford&aposs controversial red card breathed new life into the arena.
United are likely to find the atmosphere even more hostile and raucous in Istanbul. This is a contest, though often a clich, that genuinely could be dictated by the crowd.
With that in mind, plus not forgetting the fact United desperately need to secure all three points, you cannot be over-critical of Ten Hag if he decides to re-employ Amrabat at the base of the midfield, giving his team an added sense of bite and experience in front of the back-four. But based on the standard that Mainoo set, you would probably be foolish to bet against him losing his place.
Hinting that Mainoo could start again in Turkey, Ten Hag, addressing the media on Tuesday lunchtime, said: “No. We don&apost have fear. If players are good enough, they are old enough.”
With that statement in mind, it is hard to imagine United taking to the field a few minutes before 5.45pm UK time on Wednesday without Mainoo in their starting line-up. His performance against Everton was one that sent out a statement to Ten Hag and the rest of United&aposs squad.