London  - Manchester United manager Sir Alex Fergson says he is worried defender Rio Ferdinand's injury problems may be more serious than first thought.

The England defender will miss Sunday's top of the table clash with Chelsea because of a calf injury but Ferguson believes the injury could be coming from his back.

"There has been no improvement," Ferguson said. "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."

United are already without the suspended Gary Neville for the visit to Chelsea but Nemanja Vidic returns to the squad after injury.

Chelsea are two points clear of champions United at the top of the English Premier League.(dpa)