Robben out for up to two months
Club doctor Hans-Wilhelm Mueller-Wohlfahrt told Bayern's website said the injury had shown up during a routine pre-season medical and he could be out of action for up to two months.
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"This diagnosis is naturally a shock," said Robben, who helped the Netherlands to reach the World Cup final last month.
Robben helped Bayern win the Bundesliga and Cup double in his first season with the club and they also reached the Champions League final.