Luckily Man United only faced a weak team in that condition because if they faced a much stronger team then I am sure they would have suffered a deep defeat. However, this is the best result that Man United has gotten because even though they played with 10 men they were able to get three points and there is still a lot of work that Amorim may have to do considering Man United's inconsistent performance in the last few matches.
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.