LAS VEGAS — Jon Jones struggled for words while his tears flowed. Frustration and fear were evident on his face.

Although the man widely recognized as the world's most talented mixed martial artist denied using any performance-enhancing substances, he couldn't explain the positive drug test that knocked him out of UFC 200's main event and threw his tumultuous career into crisis again.

The UFC interim light heavyweight champion repeatedly cried at a hastily arranged news conference Thursday, a day after his rematch with Daniel Cormier was cancelled.

"I would never take anything that would enhance my game," Jones said. "Being labeled as someone who would ever cheat, it hurts me more than anything I've ever been through in my career."

Jones' manager, Malki Kawa, said the fighter was informed of a possible PED violation by USADA, which administers the UFC's anti-drug policy. They hoped to get the results of a test of Jones' backup sample later Thursday, although they have little hope the result will change.

If the positive test is confirmed, Jones could face a multiyear suspension. Jones and Kawa declined to name the substance for which Jones was flagged, although Kawa said it "could be" related to a nutritional supplement.

Jones said he "can't even pronounce it. I've never even heard of it."

Jones said he has been taking the same supplements for most of his career, and he has no idea why he failed last month's test after passing seven doping tests this year. Although Jones had endured numerous public embarrassments over his apparent use of recreational drugs, he has been a vocal opponent of performance-enhancing substances in MMA.

"You can say whatever you want about Jon over the years," Kawa said. "For the last year, he's done nothing but try his best to live his life right, to eat right, to do the right things, and obviously not to be in this type of predicament. ... He didn't cheat at all."

USADA spokesman Ryan Madden confirmed the potential violation stemmed from an out-of-competition test on June 16.

"Mr. Jones' B-sample is currently being analyzed by the independent WADA-accredited laboratory in Salt Lake City, Utah," Madden said. "We have requested that the laboratory report those results to us as soon as possible."

Brock Lesnar's comeback bout against heavyweight knockout artist Mark Hunt was initially named UFC 200's new main event. While fans arrived at T-Mobile Arena to watch open workouts for Hunt and Lesnar, workers tore down oversized posters of Jones and Cormier on the facade.

"I feel a little bad for (Cormier)," Lesnar said. "Guys go hard. They go through training camps and put lots of time in, and that's where D.C. really gets the short end of the stick. It's really unprofessional of anybody of this calibre when something like that happens."

The promotion changed its mind Thursday night, moving bantamweight champion Miesha Tate's defence against Amanda Nunes to the main event.

Anderson Silva also agreed to take Jones' place against Cormier on two days' notice, albeit in a three-round, non-title bout. Cormier will be an enormous favourite against Silva, who hasn't been training for a fight, but the light heavyweight champion won't get the career-defining bout he expected against Jones.

Jones (22-1) briefly left the stage at his news conference after being overcome with emotion. Several minutes later, he broke down when asked what he would say to Cormier, his longtime rival and frequent verbal sparring partner.

"I know this fight meant a lot to him," Jones said. "The fight meant a lot to me. ... I know some good will come from this, but right now it's hard to see it."

Jones' talent is indisputable after winning the 205-pound title in 2011 and defending it eight times. But he has endured numerous troubles of his own making in recent years.

He has failed drug tests around two of his last three fights. He tested positive for cocaine use shortly before beating Cormier in their first meeting in January 2015, but was allowed to fight because the detected metabolite was not banned for out-of-competition use by the Nevada Athletic Commission.

A few months later, Jones was suspended by the UFC and had his title stripped due to his involvement in a hit-and-run accident in Albuquerque, New Mexico, where he lives and trains.

Jones, who turns 29 later this month, realizes he could have another extended absence from his sport. He has fought just three times since September 2013.

"If I do have to sit out for two years, I'll definitely be back," Jones said. "I'm already thinking about the good that can happen. I try to stay optimistic. At the end of the day, I'm a fighter, and even though I may seem broken up here, I'm not broken. I'm just really upset."