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.
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 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.
"Last year the point we got against Sunderland after we were 3-1 down with five minutes to go, and we equalised in the end with (Aleksandar) Kolarov, was an important point in the league.
"I think we need to remember that 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."
Mancini must lift the mood after the Dortmund encounter, which left City with just a point from their opening two games in Champions League Group D.
That is the same position they were in at the same stage last season, when they failed to qualify from their group.
The defensive performance was also a concern with City indebted to the brilliant Joe Hart for making several world-class saves to keep the Germans at bay.
City then only claimed a draw after Balotelli tucked home a controversial last-minute penalty.
Mancini said: "It is finished now, this game. We have two days to recover for the next game.
"Last year Sunderland took four points against us and it is important we know this will be a tough game."
Mancini could restore England international Joleon Lescott in central defence having left him on the bench - not for the first time in a key fixture this season - on Wednesday.
Mancini said: "We play another game in two days and now we have four or five players fresh. Joleon is an important player for us."