The Arsenal captain was a Manchester City target before making the £24 million (Dh138m) move to Old Trafford where he will join Wayne Rooney, Javier Hernandez, Danny Welbeck in Sir Alex Ferguson's new-look attack.
"They have the best four strikers in the world," Mancini said.
"For this reason they are favourites for the title."
"Last year our four strikers did better than other strikers," Mancini said. "If we want to do better than last year then we have to work harder.
"This year, United with Van Persie are better."
Mancini had already attempted to portray his side as outsiders in the title race even before United completed their biggest signing for four years.
The City manager insisted they are way ahead of his side.
"I think without Van Persie they are two yards better than us. With him, it is four or five yards," he said.
Mancini also admitted defeat in his bid to sign Liverpool defender Daniel Agger and Roma midfielder Daniele de Rossi while accepting that Athletic Bilbao's versatile Spain international Javi Martinez is also unlikely to arrive at Etihad Stadium.
"This is a situation that is not easy because there are top clubs who want Martinez," he said.
"Sometimes it is not enough maybe to have money to spend.
"Sometimes you should move very quickly because when there are three or four top teams involved it is difficult."
However, the City manager did his best to insist that his relationship with football administrator Brian Marwood is fine, despite complaining a week earlier about the lack of additions.
Since then, Everton's Jack Rodwell has become City's first summer signing and Mancini said: "It is good, he bought Jack.
"I don't have a problem with Brian."
Mancini has also warned his side it will be harder to retain the title than it was to win it last season, saying that City will become a prize scalp and that their rivals have responded in the transfer market.
"In the second year all the other teams that play with you for the title always buy new players and they improve as a team," he explained.
"Every team wants to beat you and every team will be very difficult."
City begin the defence of their title against Southampton on Sunday when they will be without the injured Gareth Barry and Micah Richards.
Joe Hart is a doubt so Costel Pantilimon could continue in goal.
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