If not ruin him, then certainly strip this mouthy millionaire of an aura which, apart from financing the BMWs and 'tree piece suits, has also made him the envy of every other fighter on the planet.
A man taking MMA mainstream.
So, again, was this how it ends?
Nup.
Despite being famous for all kinds of flash, McGregor dug up that most mystic of athletic traits - the second wind - to score an exhausting, and unforgettable, points decision at UFC 202 in Las Vegas.
After dominating the opening two rounds, where he cut, rocked and dropped his arch rival three times, McGregor suddenly stopped dead in the third.
Not only gassed, but copping plenty from the Stockton brawler who, only five months earlier, had delivered his first loss inside the Octagon.
And now, despite bleeding heavily from cuts, Diaz looked ready to shock the world again.
Yet what came next ... well, it was the sort of recovery that defines your legacy.
McGregor not only taking the fourth, but hanging on in the fifth as his rival chased, pressured, bled.
And, sure, some still think Diaz got it. One of three ringside judges even scoring a draw.
Yet the other two both had it 48-47, giving McGregor the most emphatic of wins.
So how long now until the trilogy?
For apart from explaining afterwards how his preparation had been hampered by knee and rib injuries, Diaz also said he "wouldn't be doing shit" until the pair were signed for round three.
The mumbling fighter then going on to call himself McGregor's sensei, smoke from a vape pen and cackle: "I'm going to the after-party, he's going to the hospital".
Indeed, when McGregor had fronted the same media pack earlier, he did so on crutches. Hosing down reports of a broken foot, but revealing his shin was sore given how often it thwacked that lead leg of Diaz.
McGregor said he too was up for a third fight, but at the lighter weight of 155 pounds.
The Irishman, who received $3 million for the bout, also praised the toughness of his rival, spoke of bringing the UFC "up a level" and admitted to being fired up by the joy others had taken from his first Octagon loss.
"So this one," he said, "it was a hell of an important fight for me".