A costly error from the England No.1 led to the Saints' second goal on Saturday as they triumphed 3-1 at St Mary's in a surprise upset.
Hart fumbled his save of Rickie Lambert's shot to allow Steven Davis to bundle home the rebound as City's Premier League title hopes were dealt a hefty blow.
He had also failed to clear in the build-up to Jason Puncheon's ninth-minute opener, allowing the Saints man to stab home.
The 25-year-old keeper has made several high-profile errors this season and Mancini has hit out at the shot-stopper.
He was quoted as saying in The Sun: "Joe has made too many mistakes. He has not been good enough.
"It was a really bad mistake from Joe.
"It was not good enough. It's not strange because he's done a few mistakes this year."
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