Mancini has no doubt Hart, who is first choice for club and country, remains the best goalkeeper in England, but the Italian says he would not be afraid to drop the 25-year-old if he was consistently underperforming.
Mancini said: "For me he is the first choice but if he does not play well I change. I don't have any problem. Joe is like Mario (Balotelli), like all the other players. For England I think it will be the same. If he does not play well maybe the manager can change the keeper."
As recently as last month Hart was being hailed as the world's best after a stunning display against Borussia Dortmund in the Champions League.
Since then he has been at fault for a handful of goals and scrutiny of his form increased after his performance in England's 4-2 friendly loss to a Zlatan Ibrahimovic-inspired Sweden in midweek.
Former Manchester United star Roy Keane, working as a TV pundit, recently described Hart as "a little bit too cocky" and suggested a lack of competition for his City and England places was breeding complacency.
Mancini added: "Costel (Pantilimon) is a good keeper and if Joe does not play (well) for five or six games, probably I can change the keeper. This is normal. But I trust totally in Joe, for me he is the best keeper in England. I am sure he can do some mistakes sometimes, this is a keeper's life.
"If a keeper does a mistake, you concede a goal and you lose. If the striker doesn't score a goal the game can finish 0-0. Ibrahimovic can score against every keeper. Ibrahimovic for me is one of the top five strikers in the world."
Mancini showed great faith in Hart at the start of the 2010-11 season when he elevated him above Shay Given, an established star of the Premier League. And he now expects him to keep working hard to maintain his position.
Mancini added: "Two years ago when Joe was young a lot of people wanted Shay in goal but I decided this because I thought Joe could become one of the top keepers in the world. Joe is young and needs to work very hard. Only like this can you improve your performance."