The striker started his ban by missing the 4-0 victory over West Brom on Wednesday night as City cut the gap to Premier League leaders United to five points.
Balotelli had been in the news as he escaped punishment from the Football Association for a studs-up challenge on Arsenal midfielder Alex Song - which happened before he was dismissed late on in Sunday's clash.
Mancini had previously said Balotelli wouldn't play again this season but confirmed on Wednesday night that was because he thought the 21-year-old would receive an extended ban.
"It is important Mario takes only three games - I thought he could take six or seven games (suspension).
"Yes, for the Manchester United game he will be ready."
Carlos Tevez bagged his first goal for 11 months in the success against the Baggies and Mancini was happy to see the striker back on the scoresheet.
"I am pleased with all the players because it was important to come back and play like one month ago and to score four goals.
"I am happy for Carlos because he scored a goal. He is not ready to play 90 minutes, it is difficult.
"If Carlos continues to play he can improve his performance."