City were also superb further up the field. Antoine Semenyo and Erling Haaland were keeping the ball when it was played up to them, as well as winning free-kicks or beating people.
When he came on for Rayan Cherki, Iliman Ndiaye again did all of that brilliantly. He gave them a different dimension, and was a big reason they won the game.
That substitution was a clever one by Maresca and brave too, because while Ndiaye is better than Cherki defensively, he is also a ball-carrier who travels with the ball better too.
I remember Maresca making a couple of negative substitutions this time last year when his Chelsea side went down to 10 men early on at Old Trafford.
They ended up losing the game and he got a bit of criticism for it, but this time he went with Ndiaye, and it worked a treat.
He gave them an outlet and made some great runs – he was part of everything they did positively from an attacking point of view and was even involved in the goal – but also did his share of defending, and winning the ball back.
As the game went on, it looked like United were the ones who looked tired despite having the extra man for so long.
Maybe that was down to them playing on Thursday night, while City had their Champions League game on Tuesday, but it was not just energy that United were lacking.
They simply did not have the know-how to cause City problems. Let’s not forget Donnarumma’s late save because it was a brilliant stop, but Maresca’s men were comfortable most of the time.
Danny Murphy was speaking to BBC Sport’s Chris Bevan.