Marcellus Wiley -- Herschel Walker Is Too Damn Old to Fight!

Marcellus Wiley thinks 51-year-old Herschel Walker is CRAZY for thinking he can make an MMA comeback ... because as the old saying goes, "Father Time is undefeated."

Our photog spotted the former NFL star (and current ESPN Radio host) outside of Pink Taco this weekend. After first giving his blessing to Miley Cyrus and her burgeoning twerking career ... Wiley put the kibosh on Walker's fighting one.

As Wiley put it, "Let it go. I would hate to see Herschel Walker make it through all he's been through and now get crushed in UFC."

But Wiley did say HE could dominate the UFC and even threw down a challenge to a certain UFC star no one should ever mess with.

Tread carefully, Marcellus ...