And Paul Scholes will be sidelined for at least 10 days with his calf problem.
Rooney, who has been short of form and goals all season, was substituted with half an hour left against Bolton in the Premier League on Sunday and did not travel with the United squad for their Champions League match against Valencia tomorrow night.
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.
It could be argued it is just what Rooney needs after an awful period, during which time his private life has been thrown into turmoil and his form has collapsed.
He has still only scored once this season - from the penalty spot - compared with three in a week for Michael Owen and seven so far this season for top scorer Dimitar Berbatov.
Former England skipper Kevin Keegan claimed on Sunday that Rooney's confidence was "shot", adding the forward only had himself to blame for the press intrusion into his life because of the number of endorsements he current has.
"It is a straightforward ankle injury," said Ferguson. "And it is not the same one he did in Munich."
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