Balotelli out of Europa play-off
Balotelli, who marked his debut by scoring the winner from the bench in City's 1-0 away leg victory against the Romanian side last week, picked up the injury during his cameo role.
"Mario has a problem in his knee, not much of one, but he won't be available for Timisoara. It's important that we try to get him ready for the (August 29) Sunderland game," City manager Roberto Mancini told the club's official website on Wednesday.
The 20-year-old Italian international joined City on a five-year deal from Serie A champions Inter Milan earlier this month for a fee of 24 million pounds, according to British media.
"He is settling in well but he needs time. It's not easy to change clubs or meet new team mates but I know he's happy here already," Mancini added.
City have problems in defence for Thursday's match, with full-backs Wayne Bridge, Alex Kolarov and Jerome Boateng all injured.