The UFC lightweight champion insisted that referee Robert Byrd was wrong to bring the contest to a close in round 10 and hand victory to the undefeated boxer.
"[Mayweather is] not that fast, he's not that powerful, but boy is he composed in there," BBC Sport quotes McGregor as saying. "I thought it was close and I thought it was a bit of an early stoppage. I was just a little fatigued.
"I have to give it to him, that's what 50 pro fights will do for you."
The Irishman, who has refused to rule out a return to the boxing ring, held his own during the opening rounds but had succumb to fatigue by round nine and offered little resistance in the 10th.
The victory for Mayweather will see him return to retirement unbeaten in 50 professional bouts.