The England star has contracted a virus on the club's pre-season trip to Nigeria.
United played four friendlies, including one against Community Shield opponents Portsmouth, on the tour but it now looks to have cost them the services of their leading striker.Manager Sir Alex Ferguson said: "I doubt we'll get Rooney fit for the start of the season with the virus he's had.
Ferguson confirmed that Rooney was not suffering from Malaria, against which the entire squad had been treated before the trip.
Rooney is also likely to miss United's Barclays Premier League opener against Newcastle United the following week.
It is deeply frustrating for Rooney, as he got injuired against Reading in the start of the last season.