Rooney set for extended spell out
The striker, who ended intense speculation about his future by signing a new five-year contract last week, is on holiday as he recovers from the injury he picked up in training.
Asked if the prognosis for his return was still three weeks, Ferguson said: "I think it may be longer."
"Wayne is away at the moment, so there is no recovery," he added.
"He did his remedial work before he went away but thereafter we are happy that he is resting."
Rooney, who scored 34 goals for United last season, has struggled for form this campaign, finding the net only twice in 11 games for club and country.
He has not played for United since their 2-2 draw with West Bromwich Albion on October 16 and is now a doubt for the derby clash with Manchester City on November 10.
Rooney signed a new five-year contract with the club last week after initially saying he wanted to leave after six years.