Yes, that's true, but when we meet in the Champions League later, I'm still sure that Manchester United can still give a draw against Real Madrid, so it might make it difficult for Real Madrid to win like this pre-season game.
Ancelotti can definitely give the best for Real Madrid so that in the next match against Barcelona there is still a high chance of winning for Real Madrid.
I don't know if many agree or not, but for Real Madrid vs Barcelona in this pre-season I assume Real Madrid can win.
What you say seems contradictory and if you say Real madrid is difficult to fight Manchstr United in the champions league it is a big mistake because from experience Real madrid is superior and Manchstr United does not have strong reliable players to be able to compete with Real madrid.
You can't take this pre-season match seriously because it just a trial match so you shouldn't take the match seriously and regarding the Barcelona match with Real Madrid, most people choose Real Madrid to win but something can happen and my prediction is Barcelona win.