The England international picked up a knock during a training session only hours after touching down in Bangkok and is expected to be sidelined for around a month
The United striker had flown out to Thailand with the rest of the squad where he was expected to feature in David Moyes' first game in charge against Singha All-Star XI.
However, the 27-year-old has become the latest injury concern to hit the Premier League champion following a back problem picked up by Nemanja Vidic, and Rooney will now fly back to England for treatment.
“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,” read a statement on the club's website.
United's medical team expects Rooney to be unavailable for selection for a month, which puts his participation in the club's opening game of the Premier League season in doubt.