UFC fighter David Branch ain't sweatin' that Luke Rockhold threatened to put him in the hospital ... telling TMZ Sports his superstar opponent ain't the badass he says he is.

"He's not the killer that everybody's making him up to be. That's only what he likes to put inside of people's minds," Branch told us.

Branch heard our convo with Luke -- where the former champ said he was gonna "break his legs, rupture his liver and rattle his skull" -- and told us that crap won't faze him at all when they throw down at UFC Fight Night 116 on Saturday.

David's been through the wringer -- grindin' 6 YEARS to get back to the UFC ... so he sounds dead serious when he says Rockhold's just another guy he's gotta wreck to get the 185-pound strap.

I'M GONNA BEAT HIS FACE IN
TMZSports.com