Balotelli joined AC Milan from City for £19million last month.
Of his compatriot, Mancini told the Guardian: "Mario not being here, that must be a big problem for the paparazzi and The Sun.
"I'm happy for him that he's scoring goals for Milan now. I just think that Mario didn't understand that, for him, Manchester City was a big, big chance.
"City in the next five to 10 years can be the top club in the world. He didn't think about this, he didn't think about his future.
"He's a good guy. He's not 16, true, but 22 is still young and when you're young you make mistakes."