Matheus Cunha struck for United, but they were undone by a double from Morgan Rogers.
Afterwards, Amorim said: "If you look to all of the game we controlled the game, we played well, we create our chances, and two moments from one player decide the game. We tried to press high. Every time into Martinez we tried to press, we left our players one against one in behind - of course we cannot block every second ball off transition but we played well. I think the best team today lost.
"The amount of times that we reached near the box with space and time in a good situation to score, we are doing that. That was no different to the last game, so we are being consistent on that. I think we defended well today against a very good team. We did a good game but in the end it doesn't matter because we lost."
Skipper Bruno Fernandes was forced off with injury at halftime.
Amorim continued, "Bruno has an injury that we need to assess, I think he's not going to play for a few games but we'll see. Last year we had the same situation at this time so it is not going to kill us, it is going to make us stronger."
Amorim handed a start to Ayden Heaven at the back and also gave debuts in the second-half to fellow youngsters Shea Lacey and Jack Fletcher.
He also stated, "The kids did well today, and we have to work with the players that we have, no excuses, and try to win the next one.
"Sometimes as a manager you want to protect the kids and put them in situations where they are not going to suffer too much. Today we needed them, they did really well. They are going to improve, that is a good point today, but again we need to win games.
"I'm really careful with the kids. The way you do with your kids and I do with mine, it is not just good things. You also need to say different things in different moments. I just want to help the kids like they are my kids. I have a big responsibility here and sometimes I have to say what I think they need, not what I think they want