Van Persie completed his £24 million switch from Arsenal on Friday, and claimed he turned down an offer from Manchester City and opted to move to Old Trafford instead.
The 29-year-old Dutchman joins Wayne Rooney, Danny Welbeck and Javier Hernandez at United, and Mancini believes that line-up is unparalleled in world football, despite having the considerable talents of Sergio Aguero, Carlos Tevez, Mario Balotelli and Edin Dzeko at his own disposal.
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"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.
"I think it is the best four strikers in the world."
Mancini is no stranger to mind games and putting pressure on his rivals. Last season he repeatedly claimed his side were out of the title race, only to go on and win it with virtually the final kick of the season. So it should come as no surprise that the Italian has yet again tipped arch-rivals United for the title.
He said: "I was serious when I said this last week because United have played for the title always, every year for 20 years.
"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."