Manchester United have no idea when Rio Ferdinand will be fit, admits Fergie

Sir Alex Ferguson has admitted he has no idea when Rio Ferdinand will return from injury for Manchester United.

Ferdinand has been out for the past two weeks with a calf injury but United boss Fergie admitted the problem could be related to the defender's long-standing back issues.

Confirming Ferdinand will miss United's trip to Chelsea on Sunday, Fergie said: "With Rio we don't know exactly where the calf injury is coming from.

"We don't know whether it's coming from the lower back or whether it's a nerve thing.

"Calf injuries are usually two to three weeks, but there's no sign of him recovering from this injury.

"We're looking at the whole situation, whether or not it's coming from the back, but in the meantime he's out.

"There's no improvement, put it that way, so we're looking at something different."

On the positive side for United, Nemanja Vidic has trained and will return to central defence against Chelsea, where he will partner Wes Brown or Jonny Evans.

Watch Fergie's pre-match press conference here (video)