Joe Pytka -- the director of Michael Jordan's original "Space Jam" -- is going off on LeBron James' reboot, calling it an uninteresting mess ... and that's about the nicest thing he has to say.
Pytka -- who's also known for directing popular music videos and commercials in the '80s and '90s, including the "I Am Tiger Woods" ad -- tells TMZ ... "Space Jam: A New Legacy" is so boring, he couldn't finish it in one sitting.