The 19-year-old from Italian club Fiorentina on transfer deadline day.
The Serbia international centre-back was amazed when it emerged the Premier League champions were interested in acquiring his talents and the youngster cannot hide his excitement.
"I'm so happy, I just can't put it into words," Nastasic told Mondo.
"I have become part of a great club. Manchester City are going places and I'm now part of it. It feels truly great.
"I was really surprised when City called me and then everything happened so quickly I couldn't believe it. I'm living my dreams."
But the defender knows that the dream can turn into a nightmare if he does not rise to the challenge.
Stefan Savic moved in the opposite direction as part of the deal taking Nastasic to City after despite arriving with a big reputation.
And his young replacement is well aware of the task that lies ahead. He added: "I hope that I will not need a lot of time to adapt but I know that it is not the same to play in Italy as England.
"This is a big step ahead in my career. I'm only 19 years old."
Source: team talk
Source: team talk