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.

In fact, he says it took him 5 separate sessions to complete its 2-hour runtime.

The director has several issues with the reboot, starting with LBJ. Pytka points out when "Space Jam" was made in 1996, MJ wasn't just the best basketball player ... he was the biggest celebrity in the world.

Joe says LeBron's an incredible athlete and a very good actor, but adds ... "The truth is that LeBron ain't Michael." On top of that, Pytka says his film tied the plot to MJ's personal life -- retiring from hoops to play baseball -- while the new movie's missing a personal connection to LeBron.

Pytka's just getting started ... he feels the 1996 "Space Jam" featured a far superior supporting cast with Charles Barkley, Muggsy Bogues, Shawn Bradley and Bill Murray. In 'New Legacy' ... he says he can't even remember one thing Anthony Davis, Damian Lillard or the others did.

The director also thinks the reboot's soundtrack is "insignificant" while the OG's is classic, but his biggest beef is what happened to Bugs Bunny. No spoilers here, but Joe sees Bugs' role in 'New Legacy' as "heartbreaking."

Pytka says this new BB has no connection to previous depictions -- something they paid close attention to with the OG "Space Jam." In short, he says the new version "looked like one of those fluffy dolls you buy at an airport gift shop to bring your kid when your business trip has taken too long."

Take all this with a grain of salt, because Joe acknowledges his "Space Jam" was mostly panned by critics -- just like this reboot -- but it still became a cult classic.