The capture of last term's Golden Boot winner represents a bold statement of intent on the part of United manager Sir Alex Ferguson, who is on a mission in 2012-13 to reclaim the Premier League crown his side were pipped to in dramatic fashion by their derby rivals City in May.
A week ago Mancini had already labelled Ferguson's men as favourites to win the title and, when asked if Ferguson's men should have the favourites tag with Van Persie among their ranks, Mancini said: "Yes, for sure, and I was serious when I said this last week because United have played for the title always, every year for 20 years."
He added: "Although we won the last championship, we can't change this in one year. For this reason, I think they are favourites for the title, and now they have Van Persie - he is a really top player, the best striker last year. With Wayne Rooney, they will have one of the best couple of strikers in the Premier League."
Questions have been raised over the reported ?24million fee for Van Persie, who is 29 and has suffered with injury problems in the past, but Mancini is in no doubt that United are better off for having the Holland striker.
He said: "I think Van Persie is one of the best strikers in the world. I am sure they are better with him. Anybody who bought him would be better."
It was felt Mancini wanted to sign Van Persie himself, although he claims it was not a realistic possibility. The Italian said: "I felt he was going to stay at Arsenal or sign for another team, but not us. We already have four strikers. It was impossible for us to sign another one."
The one player City have recruited this summer is England international Jack Rodwell, who could be set for his debut in Sunday's game, a week after his transfer from Everton was announced.
Rodwell, who Mancini regards primarily as a midfield option but also as a possible one in central defence, is joining an already star-studded squad and says he is ready for the potential battle he faces to hold down a place in the team.
"It is going to be a challenge and I am up for that challenge," Rodwell said. "There are great players here and I want to learn from them and improve my game."