This is understandable because Amorim has not yet been able to stabilize this team. Still , it must also be said that the overall performance of Manchester United's attack has improved significantly compared to the last season. But Manchester United's defense is as weak as ever. Amorim has taken the necessary steps to strengthen the defense. He probably forgot about the weakness of United's defense. At the same time, Bayindir and Onana cannot be considered first-class goalkeepers. Goalkeeper Bayindir's performance was weak in all three matches. If the goalkeeper and defenders do not perform well, it is not possible to achieve good results only with good performance of the attack.
Amorim has enough defenders at the back, but they're not doing a great job, including the goal keeper. The second goal the reds conceded was the goal keeper's fault. As for the attacking force, it's incredibly fine, Mbeumo is a quality player, added with Sesko, Man U's forward is standard. He'd need to work on his goal keeper's agility and ability to make strong catch on shots, not allowing it bounce out from his palms, it was a mistake that led to the second goal.
Also, Amorim shouldn't let his frustration transfer to the players, gradually the team will pick form. Man U is too big to be winning through luck. The match almost ended a draw, and had it been so, Amorim would have walked out the pitch in a bad shape.