Mancini often tipped Johnson for big things after signing him for ?7million in 2010 but the winger grew frustrated at a lack of first-team opportunities and joined Sunderland in August.
Mancini said: "I hope he can have a good game on Saturday because I like Adam, I think he is a really good player."
The 25-year-old spoke of his frustration and of difficulties with Mancini after making the £10million move from the Barclays Premier League champions back to his native north east.
The England international will now get the chance to show City what they have lost as the Wearsiders visit Eastlands for a Saturday lunchtime fixture.
Mancini added: "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.
"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.
"He wanted to play every game and here it was difficult, he decided to leave."
Source: PA
Source: PA