Ferdinand has already been ruled out of Sunday's Premier League showdown with Chelsea at Stamford Bridge after failing to recover from a calf injury that has been plaguing him for a couple of weeks.
However, Sir Alex Ferguson now fears the injury may be related to a lower back complaint.
Poor show: Ferdinand's last game was United's defeat at Liverpool when he was easily beaten by Fernando Torres for the Reds' opening goal
'There has been no improvement,' said the United boss. 'We are having to look at something different because usually you are back from a calf injury within two or three weeks.
'But there is no sign of recovery from this injury so we are looking at whether it is to do with his lower back.'
The injury is bound to rule Ferdinand out of England's high-profile friendly against five-time World Cup winners Brazil in Qatar next week, as well as raising long-term fears for the player, who will celebrate his 31st birthday tomorrow (Saturday).
Ferdinand has featured in only four out of England's last 11 internationals and, although Fabio Capello's men are not due to play again until March, it is bound to be a concern given the central defender's importance to the Three Lions.
With Gary Neville also out through suspension, Ferguson is shorn of a large dollop experience for the Chelsea encounter.
However, there is some good news, with Nemanja Vidic ready to return after his own calf problem while Dimitar Berbatov is expected to recover from a knee injury.
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