The Italian striker is expected to be dropped to the bench for Manchester City's Boxing Day game at Newcastle.
Work to do: Manchester City's Mario Balotelli
The City manager said: 'He must improve in this situation. You need to smile to play football. That is the best thing you can do.'
Former Inter star Balotelli, 20, clearly has a high opinion of himself, saying this week that he is one of the best players in the world.
But Mancini is particular ly unimpressed with his efforts in training. 'It is good when a player has confidence in himself.
Mario has everything you need to become what he says he is already - age, physique and ability - but he needs to do a lot of work to get there.
'He must improve and he must show it. Only if you play well in every game can you say these things.
'The Premier League is so different from the Italian league. Mario must understand. If he thinks what he says he must show it in every game. At his age Lionel Messi had already won everything and proved everything.'
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