During a promotional appearance Wednesday in Toronto, Conor McGregor snatched away a bag from Floyd Mayweather that contained about $5,000. Not a good idea. Robert Guerrero could tell McGregor what happens to guys who take something that belongs to Mayweather.

Guerrero (33-5-1, 18 KOs), who is fighting the main event against Omar Figueroa (26-0-1, 18 KOs) on Saturday at NYCB Live’s Nassau Coliseum card, once held the WBC interim world welterweight title after Mayweather left the division to fight for the WBA super welterweight title. When Mayweather returned to claim what was his a year later in 2013, he scored a unanimous 12-round decision over Guerrero, who lost for only the second time in his career.

So Guerrero knows as well as anyone what UFC fighter McGregor will be up against when he makes his boxing debut in an Aug. 26 exhibition against Mayweather in Las Vegas. “I just think Mayweather is going to be way too much for him, too fast,” Guerrero said earlier this week. “Having experiences with how fast he is and how he reacts and his footwork and movements, I think it’s just way too much.

“McGregor’s a tough guy. He’s a hard puncher in the MMA world, but most MMA guys have no defense, really no movement. He’s at an amateur level in boxing. Floyd knows that, and I know McGregor knows that. He’s just a great showman.”

When Guerrero fought Mayweather, he joked that his plan was “trying to land a shot.” More seriously, he said he wanted to jab and use his footwork to get inside and pressure Mayweather. The idea was to not give him any punching room.

Reality was another story. “His reaction to what I did, I had no answer for,” Guerrero said of Mayweather. “I’d take a step, and he’s already hitting me and moving and ready to hit me again. People think he’s just fast hands and fast feet, but his reaction is lightning speed. It’s like flipping a switch. The light just comes on automatically. You’re like the skin of your teeth from hitting him, and he’s hitting you two or three times.”

Guerrero said McGregor is in for a short night unless Mayweather allows him to get close enough to land a few shots to entertain the fans. But that’s not Mayweather’s style.

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“Knowing Floyd, he ain’t going to take the chance,” said Guerrero, who plans to attend the spectacle. “He’s probably just going to blow him out of the water. Probably feel him out the first round and then blow him out.”

A more serious concern for southpaw Guerrero is the state of his own career at the age of 34 after four losses in his past six bouts. Three were against undefeated fighters — Mayweather, Keith Thurman and Danny Garcia. But after losing a split decision to David Emanuel Peralta, Guerrero took off the past 11 months, re-evaluated his approach and re-dedicated himself to training.

“You have to be able to change your game,” Guerrero said, “be able to do different things, be able to use your legs, be able to hit from angles, be able to fire different combinations and not just press forward and punch.”

Although Figueroa is coming off a 19-month injury-related layoff, Guerrero has enormous respect for a volume puncher who will be fighting for the first time as a welterweight. He knows to expect a war.

“Figueroa is in great shape all the time,” Guerrero said. “He’s one of those guys where his mindset is, ‘I’m just going to mow you down, pressure you, throw a lot of punches and see if you can handle it.’ He’s coming to fight. You’ve just got to be 100 percent prepared and ready.”