Mancini claims Nasri is playing at only 50 per cent of his capabilities.
He said: "Samir can do better than this year. He is a top player but he has not been playing at his level. Sometimes a player thinks it's enough what they did the year before and doesn't understand that every day they should improve."
Mancini added: "Samir has fantastic qualities. He should always play well.
"A player of this quality could be one of the best players in Europe. But it's not happening.
"If you are a top player you know you can improve until the last day of your career. But sometimes you get players who think it is not important to work and this is their worst mistake.
"You win the title, you think it's enough to play 50 per cent and you don't arrive the next year."