City meet their neighbours in the Community Shield at Wembley on Sunday at the start of a season in which they hope to challenge for the Barclays Premier League title. But Mancini's expensively-assembled FA Cup winners finished nine points behind champions United last term and the Italian sees them as the benchmark.
"Manchester United at the moment are on top," Mancini.
"They are a strong team, they won the Barclays Premier League and have bought four or five good players.
"I think we are very close but at the moment they are above us."
City have again added to their squad over the summer, most notably by paying a club record £38million for Argentina forward Sergio Aguero.
Defenders Gael Clichy and Stefan Savic have also been recruited along with goalkeeper Costel Pantilimon.
City will start with Aguero on the bench while last season's top scorer and talisman Carlos Tevez is still on leave following Argentina's Copa America campaign. That situation has done little to quell speculation over the 27-year-old's future.
Mancini said: "We said he needed to come back on August 5 with Zaba [Pablo Zabaletta] but it was my mistake because I told him come back on August 8.
"Carlos comes back on Monday because he has 21 days after the Copa America."
Source: PA
Source: PA