Kamaru Usman Was 'Scared' for Ben Askren After 5-Second KO

UFC champ Kamaru Usman says he was genuinely "scared" for his UFC rival, Ben Askren -- saying that 5-second, knee-to-the-head knockout looked REALLY bad in person. 

Usman was in the crowd at UFC 239 in Vegas last weekend when Jorge Masvidal turned out Askren's lights with a ferocious flying knee ... and Kamaru admits, "I was kinda scared."

"Actually, I was kinda sad cause he was out for a long, long time," Kamaru says ... "He was out for about 5 to 7 minutes." 

"I was kinda scared cause he was stiff. I thought something might've happened."

Askren was rushed to a nearby hospital -- but tweeted later that night and says he's fine. 

Of course, Kamaru and Ben have a pretty nasty history ... Askren mocked Kamaru for months by calling him "Marty," the Americanized nickname he got during his college wrestling days. 

But, when push comes to shove, Kamaru says he doesn't wish terrible things on Askren -- you can tell there's some real respect there. 

As for Masvidal, who's been pushing for a shot at Kamaru's belt, Usman says the guy needs to "SLOW DOWN" ... though he seems open to the idea.