Initially Ferguson felt that Ferdinand would only be sidelined for a couple of weeks. However, that estimate has now been revised.
Related ArticlesWigan v Man United previewLampard: United need MessiOwen already under pressure"Rio is going to be out for three to four weeks," said Ferguson.
It means Ferdinand will miss Saturday's trip to Wigan, plus next Saturday's eagerly anticipated encounter with Arsenal.
In addition, Ferdinand now seems certain to miss the next two England games, starting with a friendly against Slovenia on Sept 5, then a World Cup qualifier against Croatia four days later, when victory would book the Three Lions' place in South Africa next summer.
It also raises concerns over Ferdinand's general fitness given the £29.1 million defender suffered a back injury and a calf problem last season that kept him sidelined for extended periods.
There is some good news on the injury front for United though as Ferdinand's central defensive partner Nemanja Vidic is set to make his first start of the season at the DW stadium tomorrow as United look to bounce back from Wednesday's shock defeat at Burnley.
"Nemanja has trained with us for 10 days and he is ready for the game," said Ferguson.
"Hopefully he will be okay by tomorrow."