The England international has come in for more criticism following another mistake in today's 3-2 defeat to Cardiff City.
The Italian publicly criticised Hart on a few occasions during his tenure at the Etihad Stadium, and Platt has revealed that plans to replace him were in the works.
"If Mancini and I were still at Man City this year, Joe Hart wouldn't be there," Platt told Al Jazeera. "We made a decision to sign Asmir Begovic."
Hart played in all of City's Premier League games last season, conceding 78 goals.