'American Idol' Castoff -- Stealing Ryan Seacrest's Identity ... For Money

"American Idol" reject Shaun Kraisman's pro singing career might be off the table -- but TMZ has learned, he already has a potentially lucrative back-up plan ... impersonating Ryan Seacrest for a living.

You probably recognize him -- he's the guy from Wednesday night's episode who's a dead ringer for the "Idol" host ... right down to the way he talks. It's scary.

Shaun didn't make it to Hollywood (his audition was ... less than stellar) ... but he's unfazed -- telling us, he got such a positive reaction from his Ryan Seacrest impression on Wednesday night's show, he has decided to pursue a career as a Seacrest impersonator.

Shaun tells us, he has already booked a gig with a local radio station to do his Ryan act -- and is currently reaching out to as many people as possible to get his new career off the ground ... friends, managers, agents.