Balotelli came off the bench to inspire the Blues to a 2-0 win over Everton at the Etihad Stadium.
Until his arrival after an hour, City had been struggling to break down the Toffeemen's obdurate defence.
But Balotelli brought renewed focus to City's attack, scoring the opener, which flicked off Phil Jagielka, before fellow substitute James Milner wrapped up the points late on.
"Mario can make a big contribution this season. I am sure of this," said Mancini.
"His behaviour has been so-so. But he is a good guy.
"He likes Manchester City. He likes English football.
"Sometimes when he scores, he is unhappy. Today he was really happy. Today he knew he scored an important goal."