Manchester United boss Sir Alex Ferguson has concerns with Antonio Valencia's fitness.

The People says Valencia is giving Manchester United a fitness scare - with a mystery back injury that needs constant attention.

The Ecuadorian star has been on painkillers for the past month and Ferguson is worried that he could break down at any stage.

Valencia's fitness is being closely monitored, with docs feeling his back injury may be caused by having one leg slightly shorter than the other. He already wears a 'lift' in the sole of one boot.