His fourth bout was supposed to take place December against the top tier welterweight Leon Edwards, however, the match was rescheduled several times for coronavirus-related reasons.
The UFC flew Chimaev from Europe to the USA in February so he could receive treatment there, after saying he thought "he was going to die" because of lingering COVID effects months after testing positive.
According to MMA Fighting, Chimaev said this week: "I don't know what this disease is but it is not easily outlived."
He also posted a photograph on an Instagram story that appeared to show he had been coughing up blood.
Dana White issued a statement saying Chimaev will be back
Chimaev's social media message is at odds with a statement that the UFC boss Dana White later gave to MMA Junkie.
White said that Chimaev had been prescribed medication and was supposed to be relaxing. Instead, the Russian fighter continues to train.
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"When he got here, the doctors took care of him, and they put him on prednisone which is a nasty f------ steroid," White said.
"He's supposed to be taking this thing and chilling, relaxing, and letting himself recover. He went in and f------ trained today, felt like s---, and got super emotional and posted that.
"He's not supposed to be training, but you know, this guy's a savage. He wants to fight like every f------ weekend, and now he can't even train, so he just got emotional and posted that, but he ain't quitting."
White said Chimaev is "going to be fine."
He added: "He's training when he's supposed to be resting. He's got to stay off, at least while he's on this medication."
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Leon Edwards wishes Chimaev a speedy recovery
Leon Edwards told Insider at a virtual media roundtable event last week that he hopes his rival gets better soon.
Edwards competes March 13 against Belal Muhammad, an opponent who was brought in to replace Chimaev as the MMA fighter could not compete on the card because of his COVID issues.
The British welterweight, who himself tested positive for the coronavirus last year, said: "I had it as well and understand the effects of it.
"At the end of the day, a fight's a fight but your life and your health is more important. I wish him a speedy recovery, I wish him well, and hopefully, we can [fight] down the line."
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