Conor McGregor's boxing debut didn't at all go to plan.

The UFC double champion walked into Las Vegas with an unshakeable confidence in his ability to defeat boxing's undefeated superstar.

But, as Floyd Mayweather has proven fifty times in his career, it's hard to knock down the greatest defensive boxer who has ever lived.

McGregor's fight, which has reportedly made him over $100 million richer, ended in the 10th round after a barrage of punches rendered him useless.

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His legs were jelly, he wasn't throwing punches - and he about ready to hit the deck.

Referee Robert Byrd called the fight before McGregor was able to land on the canvas in a theatrical KO. And the Irishman was less than pleased about it.

"I thought it was an early stoppage," he said after the fight. "They should have let me keep going. Let me go down, let the man put me down."

McGregor claimed fatigue was the reason his legs were struggling to keep him upright after nearly half-an-hour in the ring.

"I get like that when I'm tired," he said. "I get a little wobbly and flowy. But f**k, get me into the corner and I'll recover and I'll come back."

But a telling new insight may have blown that claim out of the water.

Former boxing physician Darragh O'Carrol claimed there was a much more chilling reason Irealand's favourite star was looking queasy.

"Byrd's calculation to call a stoppage was likely not based on signs of fatigue, but rather signs of traumatic brain injury." he wrote in Vice's TONIC column. "Ataxia, or dizziness and loss of balance, is one of the hallmarks of concussion, a type of mild traumatic brain injury.

"Fatigue may cause sluggish and slow movements, but does not cause the imbalance and poor co-ordination exhibited by McGregor in the 10th round. Being wobbly, in the setting of pugilistic trauma, will always be treated as the result of head trauma and not as fatigue.

"To let a fighter continue on would be grossly negligent."

McGregor has a lot to reflect on before making his next move in combat sports. After visibly running out of gas in two long fights - once against Nate Diaz in the UFC and against Mayweather last month - it's clear the 29-year-old needs to work on his cardio.

"The guy has a problem," UFC middleweight champion Michael Bisping said on his Believe You Me podcast.

"He can't go the f****** distance. It's kind of an issue. In the first Nate Diaz fight, what cost him? He was tired.

"Sure, he took a good shot, that obviously diminishes your gas tank, but he was kind of gassed. "In the rematch, even though he won, he won the rematch fair and square, but still he was absolutely gassed in that. The longer the fight went the more Nate Diaz was coming back into the fight.

"It was the same here (against Mayweather). Fight started great, but he was f****. Rounds seven, eight, nine - he was a shell of himself."

UFC president Dana White said McGregor was kicking himself over the loss to Mayweather, claiming he thought he had the megafight in the bag.

"He's not [proud]," White said. "He's very upset with himself. He feels like that fight was there for the taking. He thinks that the fight was stopped early."

But White said he was happy with the finish despite having his golden child lose spectacularly.

"I was good with the stoppage. He's not a professional boxer," he said. "He stepped in here to fight arguably the best of all time, and he looked damn good doing it. I was OK with 10 rounds."