A day after arriving in the Thai capital of Bangkok, United said on Thursday that Rooney injured a hamstring in training.
The Premier League champions say on the club's website that ''it is anticipated he will be unavailable for selection for a month.''
The injury comes amid silence from Rooney about whether he still wants to leave United. Before retiring as manager in May, Alex Ferguson said he'd been told by the England forward that he wanted to join a new club.
Newly appointed manager David Moyes has not revealed any details from talks with Rooney about his future.
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