Jiri Prochazka is out of his light-heavyweight title defence against Glover Teixeira at UFC 282 next month after suffering a shoulder injury.
Prochazka, 30, has vacated the belt which will now instead contested by Jan Blachowicz and Magomed Ankalaev in Las Vegas, Nevada next month.
UFC president Dana White says it is the "worst shoulder injury UFC doctors have seen".
Czech Republic's Prochazka is expected to be out for "at least six months".
In a post on Instagram, Prochazka expressed his disappointment.
"I'll be back in six months and I'll take the title whoever holds it," he wrote.
"I apologise to my fans, friends, family who bought tickets and made plans to come to Las Vegas to support me, it means a lot to me. I also want to apologise to Glover. Victory!"
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Prochazka beat Brazil's Teixeira for the belt at UFC 275 in June and was set for his first title defence in the rematch.
White says Teixeira was offered a title fight against Ankalaev at UFC 282 after Prochazka pulled out, but he declined.
The new main event sees Poland's former champion Blachowicz, who is second in the UFC light-heavyweight rankings, face Russia's Ankalaev, who is third.
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