Rooney flew back to England in the early hours of Friday morning, having been in Thailand less than 24 hours.
"Following a scan ... on a hamstring injury sustained in training, it has been decided that Wayne Rooney should return home immediately for further assessment and rehabilitation," a statement on the Manchester United website said.
"It is anticipated he will be unavailable for selection for a month."
Though the club expects a month-long layoff, Rooney himself was playing down the injury.
"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," Rooney posted on his Facebook page.
The injury was not only a major setback for Rooney's preparation for the coming Premier League season but also a problem for new manager David Moyes as he will be prevented from experimenting with different forward combinations in framing a gameplan for the post-Alex Ferguson era.
United was already short of options up front for the Thailand game, with Rooney's fellow striker Robin van Persie on an extended break. Van Persie is due to join the team in Australia for the next leg of their preseason tour.
The absence of Rooney and van Persie means Danny Welbeck is likely to be the front man for the game against Thailand All Stars on Saturday evening at the Rajamangala National Stadium.
Chris Smalling, Ashley Young and Shinji Kagawa — who will only appear in the last Asian stop in Japan — are also out of the Bangkok game.Suggest a correction