Hart has come in for criticism - most recently for his blunder that gifted Southampton their second goal in their shock 3-1 win over City a fortnight ago.
Mancini said: "Listen, I believed in Joe when nobody else did.
"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."