The Italian has revealed his frustration after his goalkeeper's latest mistake gifted Southampton a goal in their shock 3-1 win over the Premier League champions
The England international has made a number of high-profile errors for the Premier League champions this campaign, including a mistake which led to Southampton's second goal in a 3-1 win against City on Saturday.
Hart also failed to clear in the build-up to Jason Puncheon's opener at St Mary's, a defeat that Mancini admitted has all but ended their title hopes, and the Italian has pointed the finger at his No.1 keeper.
"It was a really bad mistake from Joe," the City boss told reporters.
"It was not good enough. It's not strange because he's done a few mistakes this year."
He added: "Joe has made too many mistakes. He has not been good enough."
Hart was also criticised by Mancini after he slammed his team-mates following a 3-2 Champions League loss to Real Madrid in September.