Platt has defended Mancini over the Italian's outburst at the end of Tuesday's controversial Champions League draw with Ajax at the Etihad Stadium, claiming the incident has been exaggerated. Mancini confronted the referee after City had a late goal disallowed and were denied a penalty, and also clashed with a cameraman who filmed his on-pitch rant.
First-team coach Platt, who played alongside Mancini at Sampdoria, said: "He has always been like that - as a player he was completely like that. Monday to Friday he was fine but once he is on a pitch, if things don't go well for him - he has this fire in him. That is what he believes in."
He added: "People make a great deal about it. It is not the first time he has reacted in that way, not necessarily at Manchester City.
"He wears his heart on his sleeve. If he has got something to say he will go and say it."
Platt was speaking to media in place of Mancini at City's regular press conference to preview Sunday's Barclays Premier League game against Tottenham.
Mancini has come under increasing pressure in recent weeks with City's form - despite their unbeaten record in the Barclays Premier League - yet to reach the heights of last season.
The Ajax post-match incident came a day after Mancini lost his temper following a question from a journalist that referred to his previous statement that he was "close to seven or eight teams" last season. That in turn had been an answer to a question about a report claiming he spoke to Monaco last season.
In addition there have been reports of dressing room disharmony and speculation about the nature of Mancini's first meetings with new director of football Txiki Begiristain.
But Platt said: "We can sit here and drag it on for another three days - that's what seems to be happening to us at this moment in time; half-voices, half-truths, stories that are probably big enough to go one day are actually lasting a lot longer than that and for what reason, I don't know."
Source: PA