Every man and his dog has offered their opinion on the Conor McGregor-Floyd Mayweather fight.

But the prediction we've all been waiting for is in.

Justin Bieber - a staunch member of Team Mayweather who in the past has escorted the former pound-for-pound champion to the ring - was bailed up by a TMZ reporter this week and asked about the August 27 contest.

Bieber confirmed he's still rooting for Floyd but when asked if he expected the 40-year-old to finish the UFC champion by knockout, the pop star baulked.

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"I don't think so," Bieber said.

"I think it'll be a good fight."

A Mayweather KO/TKO remains the shortest priced method of victory with sportsbet.com.au at $1.72.

The 49-0 fighter is paying $3.50 to win by decision, while McGregor's odds - $5 for the stoppage, $21 to have his hand raised after 12 rounds - are much longer.

Meanwhile, long-time boxing promoter Bob Arum, whose days in the sport date back to Muhammad Ali's era, isn't giving the Irishman any hope.

"If you're going to get yourself in serious discussions about this guy winning, you're going to look like a fool," Arum told the LA Times.

"(McGregor) has absolutely no reason to be in the ring with a skilled, professional fighter."

Arum said McGregor should have fought against a less-credentialed boxer before facing someone of Mayweather's ability and went as far as saying he had no future in the sport.

"After he fights Mayweather, the only promoter I think who might be interested in promoting him is [small-time promoter] Artie Pelulo," Arum said.

"Who the **** would want to promote this guy?

"He can't fight for ****. Don't you understand? He's not a boxer. It'll take him years to understand.

"And there's so many more talented guys around."