Mancini often tipped Johnson for big things after signing him in 2010 but the winger grew frustrated at a lack of first-team opportunities and the 25-year-old joined Sunderland in August.
The England international will now get the chance to show City what they have lost as the Wearsiders visit Eastlands for today’s lunchtime fixture.
Mancini said: “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.
“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.”
City were well aware of the problems Sunderland can pose even before they signed Johnson.
Last season the Black Cats did more than any other side to derail City’s title bid, beating them at the Stadium of Light and then claiming a draw at Eastlands.
They were in fact only five minutes from winning that second game as well, but two late goals saw the hosts snatch a point in a 3-3 thriller in March.
Mancini said: “It is very difficult because every time Sunderland play against us they play a fantastic game.
“It is important we get three points but it will be a tough game. We need to play well.”
Mancini is not happy to be in action so early in the weekend programme after featuring in the Champions League on Wednesday.
City played out a pulsating 1-1 draw with Borussia Dortmund and the Italian does not feel the time between games is sufficient.
He said: “Now we need to recover very well because we play Saturday 12.45 — I don’t understand this.
“It is incredible that we play Wednesday night and now have two days and 12 hours to recover very well.
“We didn’t play a very good game but we lost a lot of energy in the game and we need to recover.
“Everywhere (else) the team that plays in the Champions League plays three days or four days before the game and then after, three days minimum to play another game. Here, this is strange.”
John O’Shea, meanwhile, has warned last season’s heroics against City will count for nothing this time.
The Ireland international said: “It’s a totally different situation. City have added to their squad, we have added to our squad and made some changes along the way.
“Hopefully Adam Johnson will be able to give us a bit of inside info and we can see how Mr Mancini works.
“But look, it’s a tough game for us, we are under no illusions. We will have to play very well.’’