Mancini said the 20-year-old Italian, who has scored two league goals since joining City from Inter Milan in the close season, still has some way to go before he can compare himself to the mesmerising Barcelona forward.
"I like that a player is confident in himself... (but) only if you play well every game and you score every game maybe you can say this," Mancini told a news conference.
"If he thinks what he said I think he must show (it) every game."
He also told the often moody Balotelli to lighten up.
"You need to smile to play football. That is the best thing you can do," he said.
Earlier this week when picking up an award for European under-21 player of the year, Balotelli was quoted by local media as saying: "There is only one player who is a little stronger than me - Messi. All the others are behind me."
Argentina forward Messi, the 2009 FIFA World Player of the Year, has won four La Liga titles and two Champions Leagues with Barca. He is also joint top scorer in La Liga this season with 17 goals.
Mancini's other striker to have made headlines this week is Carlos Tevez, who on Monday completed a U-turn as he committed to the club a week after handing in a written transfer request.
"I am happy we finished this situation and don't want to speak about this until the end of the season," he said.
City are third in the table, two points behind leaders Manchester United. Their next match is at Newcastle United on Sunday and Mancini said he expected defender Pablo Zabaleta to have shaken off knocks to the ankle and head in time to play.
