When Conor McGregor acts, people react.

So when the news broke late Wednesday that McGregor had obtained a boxing license in California, a frenzy of speculation began. Is McGregor — the most popular fighter in the U.F.C. — planning to leave mixed martial arts for a boxing career? Is he setting up a superfight with the undefeated — and nominally retired — Floyd Mayweather?

McGregor is rapidly developing a reputation as someone who can do anything. He was already the dominant featherweight champion of the U.F.C. when he jumped up two weight classes to split a pair of fights with Nate Diaz. He became the first fighter to hold two U.F.C. belts simultaneously with a demolition of the lightweight champion Eddie Alvarez on Nov. 12 at Madison Square Garden.

The road ahead seemed settled when the U.F.C. said he would relinquish the featherweight belt to concentrate on his lightweight belt. Fights against the titans of the division like Rafael dos Anjos, Tony Ferguson, and most intriguingly the undefeated Russian Khabib Nurmagomedov loomed.

Any card with the popular McGregor on it delivers huge audiences, and the fighter and the promotion looked to have years of big-money events to come.