The England international's future at Old Trafford has been the source of speculation in recent weeks, despite the fact that United manager David Moyes recently insisted that he was not for sale.
While he is still odds-on to remain with the Red Devils, he has been backed in this evening from 7/2 to 5/4 to join the Blues.
"We were seeing big bets at 7/2 so we cut the odds to 5/4 and the big boys were still getting involved," said William Hill spokesman Joe Crilly. "When you see a gamble like that, you have to think there is more than just hearsay involved."
Arsenal, Barcelona and Paris Saint-Germain have also been linked with Rooney's services.