Mike Williams -- BROTHER SURRENDERS ... NFL Star Claims He 'Fell on the Knife'

The younger brother of NFL star Mike Williams has turned himself in to authorities ... though Mike is now telling cops the whole thing was an accident and he "fell" on the knife his bro was holding.

Mike's brother, Eric Baylor, was taken into custody last night ... he's facing battery and domestic violence charges for allegedly stabbing the Tampa Bay Bucs receiver in the left thigh.

But cops say ... despite multiple witnesses who say the two had been arguing before the stabbing, Mike is NOT blaming his brother.

"[Mike's] account is that they were horsing around in the kitchen and he fell on the knife that his brother was holding," a rep for the Sheriff's Office says.

Suuuuuure.