He acknowledged FCSB's strengths, particularly their ability to score from set-pieces and their impressive defensive play.
Amorim stressed that United must perform at their best to secure a win in their last group stage game.
He stated on FCSB’s spot in the table: "No, it's not surprising (that they are eighth). We prove that in the Premier League - we are Manchester United but we are so low in the table. So everything can happen in football. I know they're a strong team and can score a lot of goals from set-pieces. They can win matches without much possession.
They're a really smart team and we have to perform well to win the game. We study the team to prepare for this game. Like I said, sometimes it's a team that's a little bit different in Europe.
“Without so much possession, they can score from set-pieces, with one striker who is really fast in transition. (David) Miculescu, left foot, is really strong. The two centre-backs are very strong on duels and the two full-backs go up and down until the 90 minutes. We'll be prepared for that, I'm sure my team are prepared to fight for the win."
He added on them being underdogs: "That is, of course a big difference. I cannot say a different thing as, when I was at Sporting, I said one thing so, at Manchester United I'm not going to say it doesn't matter. But it doesn't matter when the games starts. It's 11 v 11, and the other guys on the bench. We have to prove we are a better team and, for that, we have to run a lot. In competitions, especially in Europe, that can be the x-factor but we need to prove that on the field tomorrow."
And on avoiding the playoffs, he added: "It's always important, just the feeling of winning. It's really important when you have the situation of the play-offs. It's massive for us as we can clear one, two weeks without any game in the middle of the week. That is really important in this moment. Like I've said, the first thing is win the game and we need to improve the performance. We need to see our players perform to understand in each position they are better to play. So we have a lot to improve and we have to use every game and every training to see that."