Jacare Souza has tested positive for Covid-19 and is out of Saturday's controversial UFC 249 event - the first of three in seven days in Florida.
Souza, 40, was due to fight Uriah Hall in Jacksonville on a card topped by Tony Ferguson's bout with fellow American Justin Gaethje.
But the Brazilian middleweight and his two cornermen returned positive tests.
UFC president Dana White has faced criticism for staging a show during the ongoing coronavirus pandemic.
White, 50, said putting the event on was the "hardest thing he's ever tried to do", while boxing promoter Bob Arum has accused his fellow American of "cowboy behaviour".
The UFC said Souza and his cornermen were asymptomatic and left the host hotel to self-isolate, where they will be monitored remotely by a medical team.
"From their arrival earlier in the week until their departure today, Souza and his cornermen followed UFC health and safety protocols, including practicing social distancing, wearing personnel protective equipment, and self-isolating whenever possible," a UFC statement read.
"There have been no other positive Covid-19 tests reported from the remaining athletes participating in UFC 249. The response to this development is indicative of the effectiveness of the health and safety measures UFC has put in place for this event."
Hall, who will now not compete, tweeted Souza: "Brother I know it sucks. I'm sorry you have to go through this I am beyond devastated for the missed opportunity. I wish you nothing but recovery for you and your family."