Jeff Ross Prepping Kendall Jenner Jokes for Blake Griffin Roast

Nothing's off limits when Jeff Ross goes head-to-head with Blake Griffin in a Roast Battle on Saturday ... including his former relationship with Kendall Jenner.

Here's the deal ... Blake challenged Ross on social media a few days ago. Ross accepted. Now, the joke prepping begins. So does the s**t talking.

"He gets injured easily," Ross said about the NBA star ... "Maybe it'll be an emotional injury this time."

Ross won't have a hard time finding material -- but, our photog gave him a head start.

Of course, Griffin's no amateur -- he's been doing stand-up comedy for a while and Ross thinks he's a genuinely funny guy.

"Blake, to his credit, has got a lot of guts challenging me to a Roast Battle ... I think he's gonna give me a good shot."