There had been speculation for some time that Amorim would make a move to City, where Pep Guardiola's contract is set to expire next summer. However, those rumors were put to rest last week when Amorim officially signed a contract to become the new head coach of Manchester United, starting on November 11th.
It was confirmed last month that Viana, who appointed Amorim at Sporting in 2020, will become Man City's new director of football at the end of the season.
Speaking ahead of Sporting's Champions League game with City on Tuesday, Amorim revealed the prospect of following his friend to the Etihad had crossed his mind.
But the 39-year-old insisted he had no doubts about joining the red half of Manchester.
Trending"Obviously, it's something which crossed my mind but I never had any doubt," he said.
"My decision has been made, that's the club I want to work for, the context I want to be in, and where I want to continue my career.
"I didn't want anything else. It is a decision I thought about but was not undecided about it. I wanted Man Utd and that's what I did."
Amorim joked Man Utd fans will think he is the next Sir Alex Ferguson ahead of his Old Trafford move, if Sporting manage to beat City in the Champions League on Tuesday.
The head coach quipped back at News' Gary Cotterill when asked whether he could become a Man Utd hero before he even takes up his new role next week.
"I don't think about it," he said. "My focus is about winning the match, and winning it for Sporting.
"Then, the conclusions people will draw are not important for me. They could be erroneous.
"If it is a very negative outcome, the expectations will drop. That's not exactly a poor starting point to start at Man Utd.
"If we win tomorrow, people will think the new Sir Alex Ferguson has arrived - and that will be difficult to live up to!
"So I'm not interested what conclusions what people draw. I'm interested in winning the match, having a good farewell and then after that starting a new life in Man Utd.
"It could increase expectations, it could lower them. I don't know what is better for me when I start my new adventure in Manchester.
"What I want is to leave the Sporting fans and my players happy, and to qualify for the play-offs."
Midfielder Morten Hjulmund, the captain Amorim will leave behind next Monday, spoke highly of his soon-to-be former boss - and his prospects in England - and revealed the level of influence he had over his own move to Lisbon in 2023.
"Ruben is one of the reasons I came to Sporting," he said. "When you work with him, you get a clear picture of the way he sees football and how he sees you inside the team.
"That picture is very important for a player when it's clear. Then he knows his role, what he can bring to the team and to Ruben.
"That was important when I joined the club, to understand the way he plays. The way he manages the team on and off the pitch, is not anything I've seen before in my short career.
"I can't tell you all the details, but Man Utd are in good hands with Ruben."