The defending champions paid Arsenal £24 million for the 25-year-old midfielder 15 months ago but Mancini doesn't believe he has seen the best of Nasri in a City shirt.
The 48-year-old Italian says he wants to see more consistency from the Frenchman.
“In my opinion, we are talking about a top, top player," Mancini told The Mirror.
“If he wants, he can change [the course of] every game. But I think he can do better. He’s played some good games for us, but he can do better because he has got everything.”
Nasri has managed just a single goal in his last 13 appearances for Manchester City and has failed to reach the level of performance that earned him a nomination for the PFA Players' Player of the Year in the 2010-11 season - his last at Arsenal.
Mancini revealed that Nasri had been suffering from an injury in recent weeks but is fit to face Everton. He said: “Samir has had a problem with his calf in the last two games, but he’s fit now.
“And I always wait for him to play a fantastic game, as he can do this.
“When I say this, we are talking about a player like Nasri or Yaya Toure, or David Silva.
“We always think they can change every game they play because they’re capable of doing that. But sometimes, in spells when we are playing a lot of games in a short space of time – like at the moment – a player can be tired.”