Ferdinand out until end of September
The England captain, 31, missed the recent World Cup after suffering knee ligament damage on the first day of training in South Africa.
"I still think Rio won't be back until the end of September," Ferguson told reporters.
"At the time Steve McNally, the club doctor, said September. We're quite near the mark.
"It was a knee ligament injury. Not the cruciate, it was medial ligaments. It was a freak training ground accident at the World Cup.
"It was nothing to do with his back, which was a problem for him last season. But I think he should be back late September."
Ferdinand is likely to miss Premier League matches against Fulham, West Ham, Everton and Liverpool and England's Euro 2012 qualifiers against Bulgaria and Switzerland.