Mancini, who signed the France international from Arsenal in the summer of 2011, accused the 26-year-old of playing at only 50% last season.
"How I can say it without being rude," Nasri told The Telegraph. "He's a different person when he wants you and when he has you. When he wanted me, he was calling me every day, telling me everything I wanted to hear. I arrived here and for me it was a shock.
"He was really, really the opposite of Arsene Wenger. Not even a 'good morning'. Sometimes when you play he comes and talks to you. If you don't play he doesn't talk to you for three or four months. It's never his fault. You cannot have a discussion with him, try to explain something. When we have a loss, he gets crazy in the dressing room.
"He says things that you cannot say even on the street to someone. I really had the bad experience with him. He had the results as a manager everywhere he went because he had great players with him. But now, he will not be in my top three or four managers of all time."
Mancini is currently in charge of Turkish club Galatasaray.