Conor McGregor wants to fight Kamaru Usman ... something UFC legend Clay Guida believes would be an enormous mistake that could actually lead to the ending of The Notorious' MMA career -- that's how bad the UFC vet believes McGregor would get beat.

TMZ Sports caught up with 40-year-old Guida at Commerce Casino in SoCal -- where he was supporting a buddy fighting in an Alta event over the weekend (Alta takes non-fighters and puts them through a rigorous training camp, before taking an actual fight!) -- when we asked about Conor, still recovering from the broken leg sustained against Dustin Poirier, after the world's most popular MMA fighter recently declared he wanted his comeback fight to be against Usman, with the welterweight belt on the line.

"Myself versus Usman for the 170 lb. title in my comeback fight is the one I'm eyeballing at the minute," Conor said during an interview with TheMacLife.

"Why cut the weight? I've already won the 155 lb. title. I got myself to a lightweight frame, but you know, I'm big now. I feel big. I feel strong. I feel healthy. I've got good energy. I'm coming back from a gruesome injury. I don't want to be depleting myself. There should be no need to deplete myself."

When we talked to Clay, we asked him for his thoughts on a Kamaru vs. Conor fight.

"That might be his last fight. Usman is on a whole another level right now. I think that would be a poor decision, physically, for Conor McGregor," Guida says, adding "I love [McGregor], he's pure entertainment. The guy's fun to watch, but Usman is on a different level right now."

But, no chance Conor wins that fight?? Clay doesn't think so.

"I think we've all seen Usman what he's done to Tyron Woodley who I thought was unbeatable, what he did to Colby Covington who I thought was unbeatable. He's the greatest welterweight, and GSP. Greatest welterweight of all-time."

So, Clay clearly doesn't want to watch Kamaru and Conor fight. But, he's got another opponent in mind for the Nigerian Nightmare.

And, it happens to be the world's top pound-for-pound boxer.

"I would be more intrigued to see Usman fight the boxer he's calling out. Canelo. Just because it's great for both sports, why not? Just like I wanna see Henry Cejudo fight Volkanovski. Those, I think, are great for wrestling, it's great for MMA. It's great for both sports."

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We also talked to Guida -- whose 2009 fight with Diego Sanchez was inducted into the UFC Hall of Fame a few years back -- when he plans on laying down his gloves for good.

And, good news for fans of Clay ... he ain't goin' anywhere anytime soon!