When Mexico’s Gerson Guerrero (34-8, 26 KOs) had to pull out of tonight’s fight against Nonito Donaire at the last minute because of a failed eye exam, you have to figure there was a little mad scrambling going on in the  offices of Top Rank. After all, tickets were sold, pay-per-view was set, the Las Vegas Hilton was booked, and Donaire – fighting the main event – had his game plan figured out and memorized.

So where do you find a another top contender in the super-flyweight division who is in fighting shape and is willing to take a tough fight on a few hours notice?

“We were fortunate because you can find plenty of fighters who qualify in Donaire’s weight division, but that doesn’t mean they’d be ready to go in a few days notice,” said promoter Bob Arum on a last-minute conference call Thursday. “We were very, very fortunate that this kid was getting ready to fight in Mexico on Saturday. So he was in the gym and in shape and he jumped at the chance. This kid is no pushover.”

Arum said the kid will give Donaire “a very tough fight.”

Somebody better welcome Manuel “Chango” Vargas (26-4-1, 11 KOs)  of Mexico with a big handshake, a big thank you and a bigger bonus. A legitimate contender, Vargas was training for the other fight when fate pulled a fast one on him, elbowed its way into the room and made him an offer he couldn’t turn down. Switch fights? Sure. More exposure? Great. More opportunity? Why not? He might as well try to make a bigger splash on a bigger stage. You know the money has to be better.

As for three-time world champion Donaire (22-1, 14 KOs), he’s a pro. He’s used to rolling with the punches. At this level, he’s just not used to switching guys this close to the opening bell. That‘s the kind of thing that happens to club fighters and prelims, guys working their way up. It’s not suppose to happen to world champions.

“I don’t really know anything about the guy I‘m fighting, but I’m used to that,” said Donaire, who is risking his WBA interim super-flyweight title.  “Getting to where I’m at in boxing, I‘ve fought guys who I never knew until the day of (the fight). So I‘m used to fighting guys who I don‘t know anything about. I‘m going to trust in my training, and I‘m confident because I trained really hard.”

Donaire’s trainer, former IBF junior-lightweight champion Robert Garcia, also knows how the fight game works.

“It happens all the time,” he said of the last-minute change. “All fighters, before they’re superstars or sign contracts, they always go through that. It’s part of boxing.”

To Donaire, a change in opponents means he can toss away everything he had planned to use against Guerrero. It’s a whole new game, though he says he‘s learned to roll with the flow.

“There’s a lot of danger (in changing opponents) because I made my body memorize how to counter the flaws Guerrero had,” said Donaire, presently the second most popular fighter from the Philippines. “That’s what I focused on in training. I just have to change my mentality and be a smart fighter and figure out the weaknesses he has and the advantages I can take. And I trust my corner to give me the right direction.”

He said he needs impose his style on Vargas in in the early rounds, sort of lead the dance.

“He’s a taller guy like me,” Donaire said. “He’s given champions a hard time. But for me, just bring it.”

This might be one of the last fights Donaire has at super-flyweight, though WBC champion Vic Darchinyan, who Donaire knocked out in their first fight in 2007, keeps calling him out, asking for a rematch.

“He’s like a little Chihuahua, just barking and barking and barking,” Donaire said. “You try to tell him to shut up and he just keeps barking.”

Still, Donaire said he’s ready to move up in weight class right now.

“I’m ready for the bigger guys,” he said. “It’s now or never for the super flyweights, and that goes for Darchinyan and (Jorge) Arce. I can’t keep up this weight class for a long time. I expect to get up to featherweight or maybe junior lightweight.”

The fight will be one of four televised on pay-per-view.