Wayne Rooney faces a battle to be fit for the start of the new campaign after injuring a hamstring while on Manchester United's Far East tour.
The 27-year-old picked up the injury soon after the Red Devils landed in Thailand, and was quickly placed on a flight home to Manchester for further study on the extent of the knock.
A club statement said: "Following a scan this evening on a hamstring injury sustained in training, it has been decided that Wayne Rooney should return home immediately for further assessment and rehabilitation.
"It is anticipated he will be unavailable for selection for a month."
Rooney will face a battle to be fit for the start of the new season but uncertainty over his future with the Premier League champions lingers.
New manager David Moyes used his first press conference since succeeding Sir Alex Ferguson to insist the forward was not for sale but Rooney has yet to publicly commit to an Old Trafford stay.