Sorry, Baker Mayfield ... Rashad Jennings says there's NO DEBATE who the Rookie of the Year is ... and the ex-NFL star says it's "hands down" Saquon Barkley.
TMZ Sports spoke with Jennings about the tight race for ROY between guys like Mayfield, Barkley, Phillip Lindsay and Nick Chubb ... but R.J. says it ain't even close.
It really could go any way ... after each rookie has posted huge numbers this season:
Barkley -- 1,307 rushing yards, 721 receiving yards and 15 TD.
Mayfield -- 3,725 passing yards,131 rushing yards and 27 TD.
Lindsay -- 1,037 rushing yards, 241 receiving yards and 10 TD.
Chubb -- 996 rushing yards, 149 receiving yards and 10 TD.
R.J. says he predicted Barkley would win ROY last March -- before the dude was even drafted -- and he's sticking with his preseason pick.
The winner will be announced on February 2 ... until then, let the debate run wild.