Defending champions Liverpool host United at Anfield on Sunday.
Amorim said: "Sometimes things change really fast but, if you see the history of both clubs, you can feel that sometimes you have one club that is winning, winning, winning and the other club is having a bad moment.
"That happened with Liverpool when Manchester United was winning everything. And that happened with Manchester United when Liverpool was winning everything.
"So we just have to acknowledge that, to be really honest with the fans, but we can win any game. So, if we think about just winning the next game, that is the most important thing, we can win the next game.
"If we are going to fight to be at the same level of Liverpool in the future, that is the idea. I don't know how long it is going to take."