KSI says Logan Paul would be out of his damn mind to try and land a real UFC fight ... telling TMZ Sports the YouTube superstar would "get f*cked" in the Octagon.
We spoke with KSI -- who fought Logan in a YouTube superfight last month (which ended in a draw) -- about Paul's UFC pursuits ... and he says the dude's "delusional" to cross over into MMA.
"He ain't got no chance, bro," KSI says. "Anyone in the UFC would f*ck him up."
KSI's not alone ... Dana White warned Logan last week that it would be a "really bad idea" to step into the Octagon ... even though Sage Northcutt has offered to fight him.
As for their own rematch in the boxing ring ... KSI says Logan got lucky the first time, and vowed to get revenge when they fight in L.A. next year.
"Imma f*ck him up, trust."
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