Manchester United boss Sir Alex Ferguson admits he doesn't know when Rio Ferdinand will be back.

Ferdinand's recurrent issues with his back and lower leg are increasingly perplexing the club's medical staff.

"There's no improvement, put it that way," Ferguson reported. "We don't know exactly where his calf injury is coming from, whether it is coming from the lower back or 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 because of his back - but in the meantime he's out."