"Athletes are funny, but they don't always take a joke well," Ross told us at The Comedy Store in L.A.
"They're usually very sensitive."
FYI, several athletes have been featured on 'Roasts' in the past -- including Warren Sapp, Mike Tyson and Shaq.
The good news for Manning ... Ross says Peyton has already proved he has thick skin -- and expects nothing less than hilarity when #18 takes the stage.
Meantime, he has one piece of advice for Manning ... "Wear a helmet."