The 27-year old enjoyed a controversial two-and-a-half-year spell at City, where he scored 20 league goals in 54 appearances, after joining from Inter Milan for £24m in 2010.
But the Italy international has revealed he maintains a connection with City, admitting his time at the club was among the best of his career.
"I would go back to City for free. For me, the year with Manchester City has been the best one of my life."
"Honestly, at the beginning, it was difficult to live in Manchester, which is different from London, and I struggled a little bit," he added.
"But speaking of football, Manchester City are amazing. I am very happy that they have been doing so well, and I hope they will go on to win the Premier League well in advance."