The England international has made a number of high-profile errors for the Premier League champions in 2012-13, including a mistake which led to Southampton's second goal in a 3-1 win against City on Saturday.
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"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.
"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.