The blue half of Manchester snatched the elusive trophy in the last few seconds of last season courtesy of a last-gasp strike from Sergio Aguero.
Despite many tipping City to keep hold of their title, Mancini believes that following the acquisition of striker Robin van Persie, the Red Devils are likely to be on top.
"Van Persie was the best striker last year," said Mancini. "With Rooney they will have one of the best couple of strikers in the Premier League. I was serious last week when I said Manchester United were the favourites."
City begin the defence of their title with a home fixture against newly-promoted side Southampton on Sunday.