Johnson lines up against the Premier League champions on Saturday for the first time since leaving them to join the Black Cats in a £7m move.
Mancini has revealed that he was always aware of Johnson's ability, but has claimed that a move to Wearside was necessary in order for him to play more regular football.
"I think Adam played well here. The problem was maybe that he didn't play always. For young players like him it is important to play every game," the Italian said.
"I am sure he will do well for Sunderland and go to another top team because he deserves this. He is a good guy and a good player.
"I hope he can have a good game on Saturday because I like Adam, I think he is a really good player. He wanted to play every game and here it was difficult, he decided to leave."
You can read Sports Mole's full preview of the Ethiad Stadium clash here.