The 'Mighty Ducks' star tells TMZ ... he'll be in the film, "Jesus Revolution," starring Kelsey Grammer -- based on the true story of Lonnie Frisbee and the hippies he baptized in the '70s -- and the role is reflective of his own life.

Shaun tells us, someone from Lionsgate actually saw his sobriety story and wanted to offer him the role. He did a read for a director via zoom, and voila!

We're told, he plays a strung-out Vietnam vet who gets saved and sobers up. And, he's really gotten into the role ... even asking his dentist if he could temporarily take out the implants he just got, to look the part.

Well, that wasn't possible but, either way, Shaun says the role has taken him back to his own life and being drugged out ... so much so, he says, playing the role is almost like experiencing a relapse.

As we previously reported, Shaun has struggled with drug addiction for many years ... even losing most of his teeth from drug abuse. He's since turned his life around, gotten a new set of pearly whites and has been sober for 2 years!

Talk about a comeback ...