City announced on Friday afternoon that England international Johnson has signed a new four-and-a-half year deal at the Etihad Stadium. Mancini recently admitted he had been harsh in his criticism of some of the 24-year-old's recent performances but only because he believes he has the ability to become a world-class performer.
Now the former Middlesbrough prodigy, signed in a £7million deal almost two years ago, is committed until the summer of 2016, Mancini expects him to fulfil his potential. The Italian said: "Adam is a good player for me. I think he can improve a lot, though."
He added: "After signing a new contract we have time to talk regarding this. I am sure (he is going to get better)."
Johnson, who has started just six games in City's star-studded side this season, agrees there is room for improvement and wants to stay to prove himself.
There had been speculation suggesting his future might lie elsewhere but Johnson is excited by the challenge at City, who currently top the Barclays Premier League and are competing in the Champions League.
Johnson told the club's website, www.mcfc.co.uk: "I wouldn't have signed if I didn't see a big future here.
"This club's going places and I want to be up there with them at one of the top clubs in Europe. I want to become a top player, I want to keep winning things and picking up medals.
"This is such an exciting time to be at the club, things are moving so fast and there's been something new every day in the two years that I've been here.
"It's great being part of it - my game's improved, I'm playing for England and in the Champions League, that probably wouldn't have happened if I hadn't come here."
Source: PA
Source: PA