The Serbia international lost his regular spot in the side under Manuel Pellegrini and made just 13 appearances in the 2013/14 campaign.
After failing to make a single appearance in the first half of last season, Nastasic joined Schalke in January.
And the 22-year-old bemoaned the way he was treated, telling Goal: "City always have a good team with 25 top players, but the coach is only able to let 11 of them play.
"In my first year I played a lot of games. In the first half of my second season it was also good. Then I got injured and missed over three months. Then I returned and didn't play anymore.
"I spoke to Manuel Pellegrini, we've talked two or three times. He said he'd give me a chance, but I didn't feel so. It was a hard time.
"Six months before I came to Schalke, I didn't play a single game for City. I only played for the national team. I am happier to be here now."