Erik ten Hag had given minutes to peripheral and young players in the friendlies preceding that game, but he picked his strongest team to play Los Blancos and Mainoo made the starting line-up.
Mainoo had been the best player on the pitch against Arsenal in New York and he started alongside Casemiro and Mason Mount in midfield, up against Luka Modric, Jude Bellingham and Aurelien Tchouameni.
It was a chance for the younger to cut his teeth against some of the world&aposs best players in his position, but that opportunity was unfortunately cut short just a few minutes into the game when he sustained an ankle injury.
Three months of rehabilitation followed for Mainoo and he made his return to action for the Under-19s against FC Copenhagen at Leigh Sports Village, a far cry from the 80,000-capacity arenas in the States.
That was Mainoo&aposs first appearance on a pitch in public for 90 days and he was substituted at half-time, with club sources indicating that was a pre-planned decision to carefully manage his return from injury.
Just a few days later, Manoo was named on the bench for the Manchester derby and Ten Hag said: “Still a way to go, but he&aposs ready for a part of the game. That&aposs good news because he&aposs a midfielder player and we can use his capabilities, his physicality, his taking of the ball and he&aposs brave. All of his abilities and skills, he can contribute to the team.”
United played Newcastle in the Carabao Cup next and Mainoo was expected to be involved with the first-team, with many hoping he&aposd start, but he featured for the Under-21s against Salford City a day prior.
The youngsters lost 4-3 against Salford at the Peninsula Stadium and Mainoo buried his head into his shirt at the final whistle, but his performance suggested it wouldn&apost be long before supporters saw more of his face.
Mainoo was excellent and he had more influence as the game progressed. His dynamic range of passing caused problems for Salford, he recorded two assists and it was another step taken in his return to full fitness.
It was a surprise to see Mainoo with the U21s, given it seemed logical to give him an opportunity in the Carabao Cup, but a member of the coaching staff told this journalist his return from injury was being carefully handled.
That member of staff, in the same conversation after the game, agreed Mainoo had been impressive against Salford. The youngster subsequently made the first-team squad against Fulham but he was an unused substitute.
Mainoo wasn&apost spotted at Carrington in training earlier this week ahead of the Copenhagen game in the Champions League and sources confirmed that was because he was set to play for the U19s in the UEFA Youth League.
The words &aposeased back into action&apos were used and the club appear to have taken extra precautions with Mainoo to prevent rushing him back too soon from his ankle injury, which would risk aggravating the problem.
That&aposs a sensible approach, but there&aposs now just one game before the November international break begins and it would be disappointing for Mainoo not to have been involved with the first-team by that stage of the season.
Mainoo has been training for over a month now and he&aposs got two full games under his belt, so he&aposs ready for his first senior appearance of the season and Luton Town at home is the perfect fixture for his run out.
Luton are among the favourites to be relegated and make the visit to Old Trafford on Saturday afternoon. Mainoo is set to be in the matchday squad for that game and Ten Hag should give the youngster a cameo from the bench, at least.
Mainoo deserves first-team opportunities at United and when talking to people close to the player, you get the impression he possesses not only the talent but the required mentality to succeed.