Couto, 17, was on the verge of joining Barca from Coritiba in January when Man City coach Guardiola phoned him and convinced him to change his mind.
Sources told ESPN that City's financial package for the Brazilian right-back was also higher than Barca's, with the Premier League side paying an initial €6 million rising to €12m.
"I had a good conversation with Barcelona and I thought I would go there," Couto told ESPN Brasil. "But then Manchester City appeared and showed me how things would be there.
"At first, they wanted to loan me out straight away, but then I spoke with Guardiola. He told me that I would do preseason with the first team and from there it would depend on me.
"Guardiola was very attentive and said that he was really happy I was coming to the club. It was the best decision for me."
Couto was due to arrive at City in July, after turning 18, for preseason, although the coronavirus pandemic could delay his move to England.
The teenager said he leaves Coritiba with a heavy heart but the chance to succeed in Europe was too big to turn down.
"I have been a fan of Coritiba since I was a kid," he added. "My only dream when I was younger was to be a professional for Coritiba.
"I really wanted to do a full season in the Brazilian league, but unfortunately it has not worked out that way. I saw that I could achieve big things and I am fulfilling a dream by going to play in Europe."