The defeat occurred because Nottingham Forest played more optimally and Manchester United were busy building attacks but they forgot to defend. Nottingham Forest also cannot be ignored because they did beat a big team like Liverpool this season, but Manchester United's defeat should not have happened if their players could consistently maintain the defense area well because if you look at it, Manchester United's dominance is much better than Nottingham Forest.
Nottingham Forest players only had a few opportunities, but they managed to maximize them well and did not waste the opportunities that existed, that is why the coordination shown by Nottingham Forest players was much more successful in finding opportunities to score goals.
I understand that Amorim wants to play a game where they press high up, and that leaves the behind weak, because it means they are not just sitting back waiting for other teams, they are going out there and gegenpressing and that's lovely method and definitely will work eventually. But, what we have been saying about United for literally fifth season in a row right now is that they have TERRIBLE defense, their defensive players are terrible too, I would say there isn't even a single good one there to be fair, and Onana was a big terrible keeper for a while too, he looks a bit better I have to give him credit.
So the method Amorim is making them play requires faster and better defenders, and until they got some, they are not going to be any good. They need Yoro to fully heal and fit and then get another one near him this January window as well. Maybe get some amazing wing backs too, because right now, they are just using a strategy that won't work with this defense.