The Italian brought Nasri to the Etihad Stadium from Arsenal in 2011, but he believes that the 26-year-old's attitude has stopped him from taking his game to the next level.
"Samir is a player who has incredible qualities but a productivity that bears no comparison with his talent," Mancini told French magazine So Foot Junior.
"He didn't understand that you always have to give your best.
"He's a player who has the ability not to be a good player but to be a world-class player. What a pity that he won't even play in the World Cup."