A promising five-game unbeaten run came to a shuddering halt in their last home outing 10 days ago when Everton eked out a 1-0 victory against the uninspiring hosts.
United responded with a 2-1 comeback win at Crystal Palace on Sunday but failed to build on that result back at home on Thursday.
Soungoutou Magassa scored from a late corner to cancel out Diogo Dalot's second-half strike, securing West Ham a point and leaving Amorim furious.
"It's frustrating, it's angry, that's it," he said of his emotions in a curt first answer to the post-match press conference.
Pushed on how he can get the team to perform better, the Portuguese said: "It's working again and trying to win the next one."
Amorim looked the angriest he has ever been in a United press conference, snapping back at some reporters and hitting back at the suggestion United had gone backwards in recent weeks.
"It's not going backwards," he said. "The results are going backwards. We had some moments. That can happen.
"You were talking about when we had that run that team was really perfect and saying we are not that far from being in the perfect moment. We are really inconsistent.
"But if you look at the goal, 83 minutes, we have a long ball, we have everything under control and we need to do better."
Amorim plans to debrief his players on Friday when he has cooled down, unlike when he hit a giant television in the dressing room after a 3-1 loss to Brighton in January.
The United boss mentioned that incident after this latest setback, so too October's 4-2 win against the same opposition when they were made to sweat having looked in control.
"In that regard, I'm almost always consistent," he said. "I had one and you know. Brighton last year. I will talk to them tomorrow.
"I always have the feeling - I think everyone has the feeling - that we need to score more goals.
"Even with Brighton (in October) we were 3-0 (up) and then suddenly (we conceded twice). We suffer with that, we are inconsistent and we need to improve."
United return to action at winless Wolves on Monday but cannot take things for granted after failing to beat them in April last season at home.
West Ham travel to Brighton on Sunday and boss Nuno Espirito Santo said: "I think it was a good performance overall.
"First half we contain. I think the momentum of the beginning of the game was ours. We combine, we create corners, the energy was there.
"Very good second half even though we concede. I think the reaction was very positive. We dominated, we controlled, we went for it and I think it is well deserved that we got the draw."
The result left United in eighth and West Ham 18th.
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