Man United will clash with Man City in the FA Cup final on Saturday evening at the Wembley Stadium.
Speaking on The FA Cup Special Overlap, Keane admitted that he feared for Man United but claimed that writing them off could be dangerous for Man City.
“I am worried for Manchester United – Manchester City have dominated them for the past few years, but United have players like Bruno Fernandes and Marcus Rashford who can hurt City,” Keane said.
“Wembley, the atmosphere and the fact that United have been written off – it is dangerous.
“They are a bit better [than they were], but on paper, City are favourites – but that just might suit United.”