Lol Usman had Covid after the Masvidal fight. It's in that ESPN feature that came out before Conor's tweet, so it's not like it's damage control.
In August last year he was pissing blood, lost 17 lbs, and hadn't been to the gym in 6 weeks. You can't really jump into a camp for a December fight like that.
https://www.espn.com/espn/feature/story/_/page/Coverstory-April2021/the-reign-kamaru-usman
"...after he defeated
Jorge Masvidal at UFC 251 and flew back to his home in Denver, Colorado,
Kamaru Usman thought he might die. He woke up in his bed one morning and felt weak and sick. He spent the day inside trying to shake it off. He lost his appetite and hardly ate for three days.
"Man, I just made all this money and I'm not gonna be able to spend it," he thought to himself, only half kidding.
Four days later, he was so dehydrated that he was urinating blood. He lost his sense of smell and taste. On Day 5, having surmised that he might have COVID-19, he drove himself to the emergency room. After being admitted, he tested positive for COVID-19 and was prescribed antibiotics. He lived on fruit and vegetable smoothies delivered to his door and told his mother and other family members to stay away from his apartment for their protection. Eight days after that first morning of symptoms, he had lost 17 pounds."