The England goalkeeper will be working with Sinisa Mihajlovic at Toro during his loan spell.
“Every Coach has his own ideas, but I really cannot understand Guardiola's decision," Mancini told Tuttosport.
“Torino have certainly signed a great player. Despite his age, I think Joe can still improve.
“In Italy the Coaches are very competent and this will be an advantage for him. This will allow Joe to get even stronger.
“I try not to criticise other tacticians, but sending Hart away from Manchester City was a totally inexplicable choice."