The 25-year-old moved to the Etihad Stadium from Middlesbrough in January 2010 as a rising star in English football but left just 16 months later after struggling to break into the first team.
"When City come in for you, it is hard to say no," Johnson told The Telegraph. "I was in that situation a couple of years ago. I would probably advise young English players you probably won't get the chance to play as much as you would like.
"It is excellent when a club like City come for you and you're going to play for the champions, but you don't actually play for the champions. You're a squad member, it's totally different.
"Now, if I was a young lad, I probably wouldn't go to City."
Johnson is expected to line up against his former club when the Black Cats visit Manchester City on Saturday.