Rooney out for up to three weeks
The striker, who has been short of form and goals all season, was substituted with half an hour left against Bolton in the Barclays Premier League on Sunday and did not travel with the United squad for their Champions League match against Valencia on Wednesday night.
"It will be two to three weeks," said Ferguson. "But it depends because Wayne has done quite well with his recovery from injury in the past."
The injury update from Ferguson means Rooney will miss the league game at Sunderland on Saturday and also England's Euro 2012 qualifier against Montenegro on October 12.
Ferguson also confirmed that midfielder Paul Scholes will be sidelined for 10 days with a calf problem.
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