Billy Corben -- the acclaimed director behind "Cocaine Cowboys" and "The U" -- is about to debut his new film about Alex Rodriguez ... and warns the MLB star he ain't gonna like it.
The movie is called "Screwball" -- a no-holds-barred look at Alex's role in the Biogenesis scandal, when several players were accused of using performance-enhancing drugs.
A-Rod was banned for the entire 2014 season after an MLB investigation.
Corben tells TMZ Sports he's spent years working on the documentary -- and was actually contacted by Alex at one point in the process.