The Portuguese has called out four of his stars, despite their 3-1 win over Southampton.
United had to come from behind and leave it to the last 15 minutes of the game to seal the three points, thanks to three Amad Diallo goals.
Post-game, Amorim name-checked Manuel Ugarte, Kobbie Mainoo, Bruno Fernandes and Leny Yoro, stating to TNT: "It's not just tiredness but playing in this stadium we lost the last three games here.
"The pressure is different because we try to press the opponent in build up and you can feel the difficulties. Then Matheus (Fernandes) between the lines and (Tyler) Dibling between the lines created a lot of problems and we had difficulty to press high in the moment.
"This team is not used to doing that. Then it seems like lack of enthusiasm. So Ugarte is late, Kobbie Mainoo is late, Bruno going outside, the ball is inside. Then one transition and Leny Yoro cannot catch the guy. These small things push the team behind and then we get nervous without the ball.
"Then when you have one click, one goal, one opportunity - you feel the team can control the game. I want to be fair with my team. Today was not a lack of enthusiasm, it was a lack of speed and sometimes lack of control of the game."