Ferdinand suffered a knee ligament injury while in pre-World Cup training with England on June 4, but in early July it was suggested he would be fit for the opening game against Newcastle on August 16.
However, Red Devils boss Ferguson now estimates that it will take him longer than that to recover.
Sir Alex Ferguson: "Rio is probably around six weeks away from playing.
"That will also make him a major doubt for England's first two Euro 2012 qualifiers against Bulgaria and Switzerland."