Rooney suffered the injury following a tackle from Paul Scholes prior to United's Champions League victory over Bursaspor on October 20.
It appears his return to the first team will be delayed for the time being as he continues his recovery from the injury. Rooney has been in Dubai with his wife, Coleen, in recent days.
The news means that Rooney may miss the derby against Manchester City on November 10. Ferguson also rued the news that Michael Owen has suffered a hamstring injury, while Owen Hargreaves looks to have regressed yet again in his lengthy recuperation following knee surgery.
When asked if three weeks remained the prognosis for Rooney, Ferguson replied: "I think it may be longer. There is no recovery, it is just rest. Before he went away we did his remedial in terms of what we could do at the time. Thereafter, rest - we're quite happy with that.
"Ryan [Giggs] should start training on Monday, I think he's progressed quite well now. Michael Owen unfortunately picked up a hamstring injury in training. That's disappointing for him and he'll be out for four or five weeks.
"Wayne is still injured and resting, and Owen Hargreaves is still recovering from a long-term injury. I can't see [him playing in the next few weeks]. He was making very good progress when he first came back, but it seems to have stalled a little bit and he is back with the physios now.
"They're the only injuries we have at the moment and we have quite a strong squad at the moment. We hope it stays that way."
When Rooney suffered the injury his future was still in considerable doubt at Old Trafford, though he has since signed a new five-year contract to reaffirm his commitment to the club.
Rooney angered United fans when expressing a desire to leave the club and is yet to play for United since publicly questioning the club's ambition.