City meet United on Sunday evening, with Amorim struggling to find any winning consistency at United.
"Always I wish this for the manager who is in charge, but it depends on Manchester United," Guardiola said, when insisting managers need time.
"But, of course, every manager needs time, that is obvious. But in this world, you don't get much and at the big clubs, you have to win and if you don't win, you can be in trouble."
Guardiola insisted Amorim was swimming upstream coming in during the season.
He said: "It's not easy, you arrive in the middle of the season, you want to change the way you play, so it's not easy for anyone. What's going to happen at United? I'm sorry, but I don't know."
Guardiola also says tonight isn't about revenge for the defeat at home in December.
"I forget the good moments and the bad moments quick," he said.
"When we played against them, we were not at our best. The way we lost was so disappointing in the last minutes. But I'm not the type of person or manager to think about that because we've had to think we had to win seven, eight, nine, 10 games in a row to win the title.
"I will not be scared to work like that again. It's happened, it's happened this season unfortunately a little bit more than in the past."