UFC star Islam Makhachev needed an opponent after Beneil Dariush suffered a freak injury while training ... enter MMA vet Bobby Green, who took the fight on JUST 10 days' notice.
Why take a fight against a guy who hasn't lost in 6.5 years?! TMZ Sports asked Green.
"I know, I know. It's f**king crazy. Put the weight on, take the weight off, just fought, come back and fight again. I'm f**king nuts brother, this is what I do. I try to show everybody that this is real s**t. I call it G s**t, you know, doin' things that most people won't do."
Savage.
What makes it even crazier is that Green just fought on February 12 against Nasrat Haqparast at UFC 271 ... where he won via unanimous decision.
Now, Bobby will step into the Octagon with Makhachev, the man many believe will be champion before 2023 rolls around.
Islam has just 1 loss on his professional MMA record -- in October 2015 -- and has won 9 straight, including wins over Drew Dober and Dan Hooker, which IM made look easy.
But, Green isn't just stepping up to exchange an L for a fat paycheck ... he tells us how he's going to beat the 4th ranked lightweight.
We also talked to Islam ... who told us he actually respects Bobby for stepping up (and he tells us who turned down the fight).
The fellas duke it out Saturday night at UFC Fight Night.