LAS VEGAS – Sebastian Fundora has already done the unexpected by landing in the main event of Saturday’s Premier Boxing Champions’ Amazon Prime Video main event at T-Mobile Arena.

If Fundora can pull off an even greater stunner by knocking out Australia’s Tim Tszyu to capture the WBO and WBC junior middleweight belts, he is not contractually obligated to give Tszyu a rematch, Boxing Scene learned Thursday from a boxing official who reviewed the contract.

Tszyu (24-0, 19 KOs) was supposed to fight former welterweight champion Keith Thurman on Saturday, but Thurman suffered a biceps injury in training and Fundora (20-1-1, 13 KOs) was subsequently promoted from his planned pay-per-view opener to the Tszyu bout.

Fundora promoter Sampson Lewkowicz said his fighter’s shift from facing right-handed Ukrainian Serhii Bohachuk to the right-handed Tszyu is a minimal adjustment, but argued Tszyu has a hellacious task in trying to solve the 6-foot-5½ left-handed Fundora.

“Of course it is [a big adjustment],” TGB Promotions head and longtime matchmaker Tom Brown told Boxing Scene. “Some fighters don’t have a problem [adjusting to] southpaws. If you talk to my brother-in-law [Hall of Fame trainer] Joe Goossen, he would act like the world ended. He hates preparing for southpaws.”

Tszyu indeed described the change in opponent and fundamentals as “a little switch.

“I haven’t fought too many lefties, but I did a bit of work with lefties in this prep,” Tszyu said of the two-week window before Thurman’s withdrawal and fight night.

Brown is impressed by Tszyu’s unbothered take on the change.

“He’s got that old-school mentality that he’s here to fight,” Brown said. “Two weeks with [left-handed] sparring partners, he’s ready for this.”

Fundora’s lone loss came in his last fight, but Lewkowicz explains part of why that knockout loss to Brian Mendoza happened is that Fundora had so dominated the earlier rounds and had a momentary lapse of attention.

“What a change of events, huh?” Fundora asked at Thursday’s news conference, referring to his landing the Tszyu fight. “It’s a change of events I’m going to take advantage of.”

Before that, Fundora had rocketed up the rankings, and Lewkowicz said he sees his guy as a 60-40 favorite. “It’s 50-50 at the worst,” he said.

At BetMGM, Tszyu is a -600 favorite and Fundora is a 4/1 underdog to win.

The odds for Fundora to win by knockout or TKO are 9/1.

“You’ve got to applaud [Tszyu],” Brown said. “He could’ve complained about being forced into this. … He said yes immediately. We didn’t go back and look at how many southpaws he fought in the amateurs. He just accepted the fight.”

It’s what a world champion does.