The Sunday Mirror says Balotelli was left out of the City squad for yesterday's trip north to Newcastle.
Mancini made clear his anger, amid claims that the player's training had not been up to standard in the past few days.
Questioned on why Balotelli didn't travel to the north east, Mancini replied: "Mario is not in good form at the moment.
"It is better for him to train well.
"His form is not good enough to play."
Privately, the City boss now wonders whether Balotelli will ever fulfil his potential.