The Etihad Stadium club tied up a deal on transfer deadline day for around £12million, with Stefan Savic moving to Fiorentina as part of the deal.
The 19-year-old had only spent one season in Italy, and as such has revealed that his big move has come earlier than expected.
“The first I heard of it was 15 to 20 days ago and of course I was surprised,” Nastasic told The Daily Mail.
“I did not expect to transfer to a major club and league such as Manchester City and the Premier League after only a year at Fiorentina.
“Playing for City by definition means we’ll be playing for trophies and I’m sure that City intend to strive to win each and every trophy.
“This is great motivation for me and the main reason for my arrival,” he concluded.
Nastasic is being touted as the ‘next Nemanja Vidic’ and will provide competition for Joleon Lescott and Vincent Kompany in the centre of City’s defence.
By Gareth McKnight