The Dutch legend believes that David Moyes's side's "inconsistent" start to the campaign is due to the lack of signings in the summer.
"I can fully understand that new boss David Moyes has now asked the board to sanction the purchase of three top-class footballers in the winter to keep the team competing at the highest level in England and Europe," The Mirror quotes him as saying.
"I have reason to believe that funds are being made available at United. And they should because it is a necessity. And when they do buy those three stars, it will be easier to let Wayne Rooney follow his heart if he wants to leave.
"They kept basically the same team and squad in the summer, which is why they are inconsistent in the Premier League."
United are currently ninth in the league table with 10 points from seven matches.