"I think Samir has fantastic qualities," the Italian told The Guardian. "With his quality, he should always play well. Every game he could be the difference. A player of this quality could be one of the best players in Europe. But it's not happening.
"Sometimes a player thinks it's enough what they did the year before and doesn't understand that every day they should improve.
"If you are a top player you know you can improve until the last day of your career but sometimes you get players who think it is not important to work and this is their worst mistake. Samir can do better than this year. He is a top player but he has not been playing at his level."
And Mancini has told England international Hart that he must also focus all his attention on football.
"I believed in Joe when nobody else did," Mancini added. "I put him in the goal when everybody thought it was impossible that he could play ahead of Shay Given, who at that time was one of the best goalkeepers in Europe. I love Joe. If not, I wouldn't have put him in the team two years ago.
"But it's simple. If Joe continues to make mistakes, he goes on the bench.
"I've done it with Samir, (David) Silva and (Carlos) Tevez this season and it can happen with Joe. The problem is the goalkeeper makes a mistake, we have lost the game.
"Joe has the quality to be the best goalkeeper anywhere. He is the best goalkeeper in England but in the situation Manchester City are in, if you want to stay at the top, you need to work hard and think only about football. He needs to think only of his job and that is being a goalkeeper."