David Beckham -- They Call Me Tenderfoot

There's a reason David Beckham is rocking a "precautionary" walking boot on his right foot -- and it's not very precautionary at all.

TMZ has learned David is suffering from a "tender foot" -- a result of a "series of knocks" he took to the appendage after an MLS playoff match against Chivas USA earlier this month.

A rep from the L.A. Galaxy tells us the foot is already better, and he's "practicing with the team today ahead of Sunday's MLS Cup."

... whatever that is.