The 20-year-old winger left boyhood club Valencia to sign a five-year deal with the Sky Blues.
He told Marca: "Everything was done late with City. The first contact was with Valencia and they said that they wanted to speak with us. Then we reached an agreement before they negotiated with Valencia.
"I was very nervous that day. My agent was in Manchester with the club and he called me to tell me they had an agreement and I'd play for City. It's a club who want to win everything so I think I'll improve a lot here. Very big teams called but I have to keep that to myself."
On his talks with Guardiola, he added "He congratulated me and gave me peace of mind. It calmed me down and made me want to sign even more.
"[I want] to work, adapt quickly and learn. The more competition you have, the better you become. You always have to be at your best because otherwise you'll be on the bench.
"On the one hand, I'm excited and happy, but at the same time I'm sad because I'm leaving my club. Valencia game me everything and trained me since I was seven. I've had hard times there and that's why I ended up leaving, but I want to emphasise that I'm very grateful to Valencia and will always support them."