Nasri's ability has never been in doubt, with City having splashed out £24million to secure his services from Arsenal in 2011.
He impressed during his debut season at the Etihad Stadium, as he helped to land the Premier League title, but has been criticised at times this term for an apparent lack of effort.
Nasri was, however, back to his best during a crushing 4-0 victory over Newcastle United on Saturday and Mancini is confused as to why the 25-year-old cannot perform to those levels on a more regular basis.
He said: "I would like to give him a punch, because a player like him should play like this always.
"Every game. Maybe one game sometimes he can play badly.
"I don't understand what was different. I can't understand how a player with his quality doesn't play like this every game.
"The second year is always difficult to win the title again. And sometimes the player can think it is enough to play 50 per cent. We've probably had this problem this year."
Mancini is right in his assessment, with too many of City's players having allowed their standards to dip in 2012/13.
An inability to repeat their heroics from last season has left the defending champions 15 points adrift of arch-rivals Manchester United in the Premier League title race, and Mancini admits that is 'too much'.
With City set to face United in their next league outing, Mancini added: "We can't think we are 15 points behind because for me, the gap is not correct. Fifteen points is too much. I think it is six or seven, not 15.
"We showed this in the first derby when we deserved to win and lost. You can play well, but if you don't score and concede, you lose.
"They (United) have won a lot of games, difficult games, to stay on top and they deserve to win the title for this.
"This is the mentality you have when you win everything for 20 years and you know if you don't play well it's important to win."