The England striker will fly home from Bangkok just hours after arriving with his team-mates and is set to be out of action for at least four weeks.
A statement on United's official website read: "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 wrote on his Facebook page said: "Gutted to have picked up a niggling injury especially as training was going so well, heading back to Manchester to have it assessed fully but early indications are nothing to worry about."
New manager David Moyes used his first press conference since succeeding Sir Alex Ferguson to insist Rooney was not for sale but was unable to "categorically" state that the forward wanted to stay at Old Trafford.
Rooney's future was thrown into doubt towards the end of last season when Ferguson declared he had asked to leave the club - a claim the player has privately denied.
He has been linked with moves to Arsenal, Chelsea and Paris St Germain but has two years left on his current contract.
A month-long lay-off would make Rooney doubtful to face Wigan in the Community Shield at Wembley on 11 August and Swansea in United's Premier League opener on 17 August.