Amorim was heavily linked with a move to Man City during a period of uncertainty when it came to Guardiola's future, but the latter has since signed a new contract with the Citizens.
Hugo Viana will be leaving Sporting Lisbon to become Man City's new director of football next summer, and it was believed that Amorim would be targeted if Guardiola decided against penning a new deal.
However, the Portuguese has said that he was never contacted by the Citizens, and he had "no doubts" over making the move to Man United once their interest in him was confirmed.
"We never had [a conversation]. This was my only option," Amorim told reporters. "When Manchester United talked to me, I had no doubts because I already had something in my mind that it could be a possibility.
"With Manchester City or Hugo Viana, there was nothing about that."
Amorim is preparing for first Manchester derby
Amorim has already faced Man City this season, with his Sporting side recording a 4-1 victory over the Citizens in the Champions League, and Guardiola's side are currently on a run of just one win in their last 10 games in all competitions.
However, Amorim, who will take his Man United side to the Etihad Stadium on Sunday afternoon, has insisted that "great teams can respond at any moment".
"The great teams can respond at any moment. And I think they are in a better place than us in the type of understanding the game, the way they play, the confidence they have," he added.
"Even in these kind of moments, we have a lot to focus on our team. So our focus is total of course, we will have a strategy: try to win the game with the strategy like it should be. But we are focused just on our team."
Amorim backs Onana after error vs. Plzen
Amorim also insisted that he has no concerns over his goalkeeper's form heading into the Manchester derby.
Andre Onana made a huge mistake against Viktoria Plzen in the Europa League on Thursday evening, with his pass into Matthijs de Ligt being intercepted, allowing Matej Vydra to send the home side ahead.
The Cameroonian also struggled against Nottingham Forest in the Premier League last weekend, but Amorim has backed the stopper to bounce back in the derby, having previously shown that he can recover from difficult moments.
"Yeah we have to take it like it is. He had already, in the past, a situation like that. So we have to work on this. If you see the play, we are trying to build up a lot with the goalkeeper making the balls with Matta [Matthijs de Ligt] a lot of times," he added.
"This time, we have a disconnect a little bit during the play and, when we try this, we suffered a goal, a difficult goal for us in the start of the second half. But we managed today, not like against Nottingham [Forest]. We managed to help Onana, like he did in the past for us."
Man United are currently 13th in the Premier League table, boasting 19 points from their opening 15 matches, which has left them eight points behind fourth-placed Man City ahead of Sunday's contest.
Written by
Matt Law