McGregor, the once seemingly invincible Irishman destined to hold UFC titles in multiple weight classes, tapped to a rear-naked choke applied by Nate Diaz on Saturday night. The finish came at 4:12 of the second round in a welterweight bout McGregor appeared to be in control of early.
Diaz (19-10), who took the fight on 11 days' notice after McGregor's original opponent, Rafael dos Anjos, withdrew because of a broken foot, reacted as only Diaz could.
"I'm not surprised, m-----f-----," Diaz said. "There's a new king in this ... now."
McGregor (19-3), whose 15-fight winning streak was snapped, remains the UFC featherweight champion despite the loss. The bout was contested at 170 pounds because Diaz had virtually no time to cut weight. Originally, McGregor was attempting to move to the 155-pound division to challenge dos Anjos for the lightweight title.
McGregor blamed the loss, his first since 2010, on inefficiency. All three of his professional losses have come via submission.
"I took the chance in going up to 170," McGregor said. "I was inefficient with my energy. I'm humbled in victory and defeat. He took the fight on short notice and done the job. He was efficient. I was not. These things happen. I'll learn, and I'll grow. I'll face it like a man."
McGregor detailed the pitfalls he encountered against a heavier and taller opponent than he is accustomed to fighting.
"I think I needed to be a little more select, a little more efficient with my energy," McGregor told ESPN. "I needed to recognize that a bigger man -- you must put the shots together a little bit more to stop a bigger man."
McGregor had his way with Diaz in the opening round. His overhand left connected frequently with the right side of Diaz's face. Diaz was cut almost immediately and was bleeding badly over his right eye for the majority of the bout. But McGregor, who has famously boasted that no one can take his left hand, couldn't put down Diaz, who never appeared hurt.
Nevertheless, McGregor was mostly in control. Even when Diaz scored a takedown late in the first round, McGregor immediately swept him and finished in top position.
The next round had a similar feel, as McGregor landed one left hand after another. Perhaps he grew too comfortable doing so. A couple of minutes in, Diaz wobbled him with a straight left. Diaz threw his hands up and smiled after the punch landed. Once McGregor's legs buckled, Diaz immediately started to close in on him.
McGregor never went down, but Diaz pushed him to the fence and landed a series of left hands. McGregor eventually managed to push off and began throwing back, but Diaz, with his two-inch reach advantage, expertly stepped in and out of McGregor's attempts to hurt him and landed another hard jab and then a straight left hand that forced McGregor to shoot for a takedown.
Diaz immediately countered with a guillotine, flipped McGregor over and moved into full mount. McGregor was forced to turn and give up his back, which led to the rear-naked choke and tapout. It is the ninth submission win of Diaz's UFC career, second most in the promotion's history. He trails only Royce Gracie, who has 10.
According to Fightmetric, Diaz outlanded McGregor in total strikes, 77 to 61.
McGregor indicated he would likely still compete at UFC 200 on July 9 in Las Vegas, as originally planned. He said he hasn't given up on fighting for the 155-pound or even 170-pound championship down the line but will probably move back to the 145-pound division for his next fight and defend the belt he won in December in a 13-second knockout over Jose Aldo.
Asked who he felt was more deserving of a shot between Frankie Edgar and Aldo, McGregor said he would let the fans decide.
"It's hard not to give Aldo another shot. He was 10 years undefeated," McGregor said. "But then again, he pulls out a lot. He doesn't show up. Frankie at least gets in there and competes.
"I don't know. I'll keep my ear to the ground and see who the fans want to see the most, what are the fans interested in. Then I'll sit and wait patiently for the lightweight belt to be contested and I'll make my way back up."
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