Mancini's tough love approach did not work for this talented but under-used 25-year-old.
Johnson told The Sun: "What drives me on now is to play and to be appreciated.
"There are a lot of big players at City and it was my time to go and be my own player. From the last year, or longer, I wasn't playing enough.
"I was forgotten about. I want to get playing again and have people talk about me again for being good. That is why I signed for Sunderland.
"I've come here and I'll be able to play with a bit of freedom and do what I did before I left Middlesbrough."