The title holders are into the fifth round after last night's controversial 2-1 win against Leicester City.
Amorim conceded United were flat on the night and said afterwards: "I cannot focus just on saving the season with the momentum of the Cup. That's not the way I see football, it's not the way I see Manchester United.
"It's everything, it's the performance, it's the result. I'm not thinking about the end of the season now, because the final is at the end of the season. I don't care about that, I care about the moment.
"The moment is clearly not good enough. It's a good result. We move forward to the next stage. We have a week to prepare but today was not a good day."
He also said, "It's everything. The coach is the first one responsible. When a team doesn't perform and doesn't improve, it's the coach. But we are here to see the game, study the game and try to improve for the next game."