it would depend on how long the fight stayed standing, the boxer always has a chance, once it goes to the floor then it becomes the MMA fighters domain, there have been professional boxers fight MMA and have lost once they go to the floor, the grappling is a skill which becomes difficult to learn and adapt to if you spent your career boxing.
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the fight wasn't over in 3 rounds, McGregor won rounds 1,2,3...4 &5 where 50-50, 6 Mayweather, 7 was close, 8 Mayweather but still close, 9 McGregor was fatiguing visibly and Mayweather stepped up and was picking off McGregor 10. Mayweather continued from 9 and the fight got stopped.
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I thought it was a good fight, with McGregor being more competitive than most expected..... Mayweather kept turning his back which was strange and McGregor kept dipping into some MMA notably hitting Mayweather in the back of the head when Mayweather turned his back on him.
I enjoyed it but the best man won, shame as I wanted McGregor to win but thats combat sports for you.