David Hasselhoff has fans worried after he was recently spotted using a walker and looking thinner than his beefy "Baywatch" prime ... but he is doing just fine, TMZ is told.

A source close to the Hoff tells us ... pictures of DH using the walker were of him leaving physical therapy ... which the poor guy needs after having both a knee and a hip replacement.

We're told he's still active ... exercising and hiking -- so fans can relax and stop spiraling with speculation.

The concern, of course, makes sense ... the beloved star is not snapped out and about much these days -- so when he is seen not looking exactly like he did in the '90s, fans are jarred. But, again, all is well.

Bottom line ... don't get all huffy about the Hoff ... he's just doing his thing.