LAS VEGAS — Just 20 months ago, a former plumber from Dublin fought a fearsome superstar who hadn’t lost in a decade. Conor McGregor talked his way into a big-money showdown, but he had to back up his boasts against Jose Aldo, one of the greatest fighters in his sport.

McGregor finished the UFC featherweight champion in 13 seconds with one devastating left hand.

Floyd Mayweather presents an entirely different challenge when McGregor steps in the Vegas ring Saturday night. In his professional boxing debut, McGregor must figure out how to beat Mayweather at his own game — something the best boxers in the world have never done.

McGregor is a massive underdog, and deservedly so. But stop pretending the improbable doesn’t happen in combat sports, particularly when the self-proclaimed Mystic Mac is involved.

“The people who don’t believe it don’t know anything about fighting,” said UFC President Dana White, who admittedly has a slight bias here. “This isn’t going to be a boxing match. This is going to be a fight.”

It’s also a fight between a 29-year-old southpaw in his prime and a 40 1/2-year-old veteran who hasn’t fought in nearly two years and doesn’t particularly want to be here, as he acknowledged repeatedly Wednesday. Mayweather isn’t the fighter he once was, and his competitive fire is dwarfed by his need for the mammoth paycheck from this endeavor.

But how does an MMA fighter with no competitive boxing experience since childhood take on a 21-year pro veteran with world championships in five divisions and arguably the most brilliant defensive skills in the history of the sport?

He gets in Mayweather’s face, both before and during the fight.

He charges forward from the opening bell and forces Mayweather to think defensively all night.

He throws hundreds of punches from every angle with the goal of setting up the vicious left hand that has turned out the lights on MMA opponents.

And if none of that is successful, McGregor could bend the Marquess of Queensberry rules to their breaking point in hopes of goading Mayweather into a brawl.

Mayweather hasn’t seemed interested in McGregor’s typical trash talk during the buildup to the fight, but perhaps he could waver the champion’s focus with the wrong word, an errant elbow or a rabbit punch behind the head — enough to draw a warning, but not lose a point.

McGregor amplified his boasts this week, calling Mayweather “a broken man” and vowing to finish him within two rounds. Nobody knows whether McGregor actually believes his bluster, but the Irishman has defiantly vowed he will win.

It’s not probable. But with McGregor, anything is possible.