By their standards presently, every club that's normally an average team is big for them and true like you said , they could have been punished had they played against a side like Liverpool. Observed that no striker scored for Man UTD yesterday? Meanwhile they had a striker in the game. Since this season started, Rasmus Hojlund just has two goals and he was a big money spending when he was brought into the club last season from Atalanta.
Even his replacement, Joshua Zirkzee is nothing to write home about because the both just play like a bunch of confused individuals.
When you play a strong team with 10 men it is much harder to get a win because they have everything to punish Man United. But they were lucky to only play against a relatively weak team so they got the three points from the game. Man United's attack looked like it lacked quality and you could see some performances that were just plain bad because even though they had a few chances the finishing was a problem.
There is almost nothing special about Man United's attack line now because their quality is far from what is expected and I believe the quality of the players owned by Amorim is now below standard. There are statements from some observers and if you want to develop then go to another club like some Man United players who have left or been loaned.