The 51-year-old believes that manager Sir Alex Ferguson is not the reason for the Red Devils' dominance over the years, but the ability to spend big money on players.
"If you analyse the transfer record and the history ... Rooney, or Ferdinand, for example ... £30m," Benitez told Radio Merseyside. "£30m for a young player or a centre-back. Every year they are very well off.
"It's not just because of the interviews or the press conferences [with Ferguson]; it's because they had money."
Meanwhile, the Premier League champions prepare to face bitter rivals Liverpool at Anfield on Saturday.