The bold claim came from UFC middleweight title contender Chael Sonnen, who thinks the two will come to blows again before not too long.
McGregor turned his hand to boxing this year and fought Mayweather in his first professional fight.
The American came out of retirement for the multi-million-pound bout and won via knockout in the 10th round.
However, it was McGregor who won the heart of millions with his spirited performance against a man who had won all 49 of his previous clashes.
The Irish UFC lightweight champion is still to decide what his next career move will be, and he has recently been called out by Oscar De La Hoya.
But Sonnen thinks McGregor will return to MMA until Mayweather is ready for part two.
“That’s [De La Hoya call-out] a curveball from left-field that nobody saw coming,” Sonnen said on his official YouTube channel.
“I don’t think we’re going to see Conor back in that ring until Mayweather goes through his money, which is going to happen.
“It will probably take a couple of years and we are likely to see a Conor-Mayweather two regardless of how many people are going to say ‘Chael, you’re wrong on that.’
“We’ll need two to three years to see if I’m right but Mayweather will be back so if Conor’s still hot at the time, Mayweather will still be hot.
“Comeback fights are huge. Mayweather will fight again, that part I’ll tell you for sure.”