Rio Ferdinand is facing an extended period on the sidelines as Manchester United try to get to the bottom of his latest injury concern.

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, and said: "There has been no improvement. We are having to look at something different because usually you are back from a calf injury within two or three weeks."

He added: "But there is no sign of recovery from this injury so we are looking at whether it is to do with his lower back."

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