Kamaru Usman scrunched his face in a playful manner and tilted his head. He had just defended his Ultimate Fighting Championship welterweight title in a five-round brawl with Colby Covington at Madison Square Garden at U.F.C. 268.

The belt for the 170-pound class rested next to him on a table, the leather-encased gold plate shining as brightly as Usman’s metallic suit lapels and his diamond necklace. Earlier in the week, Usman solicited to fight Saúl Álvarez, the best pound-for-pound boxer who — also on Saturday night — delivered a masterful 11th-round knockout of Caleb Plant in Las Vegas. Usman’s boss, U.F.C. President Dana White, downplayed the feasibility of a crossover megafight to reporters minutes before Usman’s news conference. Usman disagreed.

“Dana doesn’t know that,” Usman said. “I want something that scares me.”

The prospect of matching Usman with the best boxer on the planet may not frighten Usman, but White and U.F.C. officials are right to be hesitant about the hypothetical bonanza. In this era of blurred lines within combat sports, organizers have forfeited competitive logic to appease a new crop of paying fans. With Usman and Álvarez, the reward may not generate the success of similar recent experiments.

Álvarez commands an audience, and he produced an unquestioned result, knocking out Plant precisely to unify the 168-pound belts. Most of the audience at the MGM Grand Garden Arena supported Álvarez, a Mexican who goes by the nickname Canelo, and Showtime built the event solely around him. People from across the country bought the pay-per-view broadcast, too, including White. A picture circulated on Twitter of him watching it on a monitor octagon-side as Rose Namajunas and Zhang Weili fought for the 115-pound title in the U.F.C.’s co-main event. White later said he had bet $100,000 on Álvarez to win by knockout.

“I watched the Canelo fight tonight,” White said when asked if Usman should box Álvarez. “He don’t want to fight Canelo. Come on, man, let’s stop this.”

This isn’t the first time White has dealt with crossover fights. He allowed the brash Irish U.F.C. fighter Conor McGregor to box Floyd Mayweather in 2017, a spectacle that profited McGregor at least $30 million. That sum is rare for mixed martial arts competitions, and while Usman most likely would receive far less for a bout with Álvarez, the amount would still be a pay raise from what he gets facing opponents in mixed martial arts.

Two brothers with popular YouTube channels, Jake Paul and Logan Paul, have led a shift in combat sports in the past year. Logan Paul fought Mayweather this summer in an exhibition while Jake Paul has beaten relatively safe opponents: a retired basketball player and two M.M.A. fighters past their primes. The specifics seemed outrageous to pure boxing fans, but the brothers’ large social media following allowed the fights to be a success.

Usman does not have stardom like the Pauls or McGregor, and he has said he does not want to pursue it. But more eyes on his fights would lead to more dollars. When asked if he would box Álvarez for charity, he laughed.

“Of course we want the money,” Usman said.

The challenge of facing Álvarez also intrigues Usman because he has beaten most of the top contenders in his division.

“Don’t get me wrong, he is a master at what he does,” Usman said. “I love it and I respect it, which is why I want to challenge myself.”

Usman is highly respected in mixed martial arts circles. After he beat Covington, White and others spoke about how he might end up comparing to the greatest welterweights of all time, like Georges St-Pierre.

Still, it is clear that he does not have the same fan following as Álvarez. Usman’s support at Madison Square Garden was mixed, and the crowd at times chanted for Covington, a fervent supporter of the former president, Donald J. Trump, who has made conservative politics a central part of his persona.

The cheers for Covington grew in the later stages of the bout, as he came back from two knockdowns in Round 2 and challenged Usman down the stretch.

Had Usman not gotten the knockdowns, the judges easily could have scored the decision differently, leaving room for debate and an easy avenue for a third fight between Usman and Covington.

And beyond Usman-Covington, other fighters also commanded attention on the U.F.C. 268 card. The lightweights Justin Gaethje and Michael Chandler offered the best clash of the night, a high-paced showdown that Gaethje won, though both men left bloody and needed hospital examinations. Namajunas also defended her strawweight championship against Zhang in a split decision.

If Usman were to box Álvarez, the differences between mixed martial arts and boxing, including the sports themselves and the pay gap between the U.F.C. and the top tier of boxing, would complicate promoting the event.

Álvarez has said he is not interested in fighting Usman, and fan desire has not reached the point yet to say otherwise. If it happens, though, Usman said he would be ready to compete in Álvarez’s preferred discipline. Usman would fight him in mixed martial arts, too, he said, though he doubts Álvarez would attempt that.

“We’re the ones that are willing to go over there and take that risk,” Usman said. “They wouldn’t dare come over here and take that risk.”