Costa faced issues with a troublesome elbow infection, necessitating multiple surgeries, and UFC President Dana White confirmed his withdrawal. Fans were left in disbelief as the co-main event, initially at risk of being canceled, has now transformed into a contest that many believe is even more exciting than the originally scheduled matchup.
One wrote: 'I love seeing fighters like Kamaru Usman stepping up to take on the tough challenges! It's what makes the sport so thrilling.
'The fight against Khamzat is going to be absolutely insane! Can't wait to see these two warriors go head-to-head in the octagon.'
Another posted: 'This is going to be the biggest fight of the year (imo). Chimaev is no pushover whatsoever but never underestimate the dog that is in 'The Nigerian Nightmare' Kamaru Usman because he is coming to fight at UFC 294. I favour Usman but don't be surprised if Khamzat wins.'
While a third added: 'Short notice older fighter, moving up in weight against the young lion. This is an EXTREME uphill battle logically, still has to travel and get adjusted to Abu Dhabi as well.
'I really want Kamaru Usman to win at all costs but this is going to be a tough one!'
Dana White revealed the news in a clip he shared online, saying: 'Costa is hurt, you know all the rumours flying around but we got it done.
'So the No 4 welterweight in the world Khamzat Chimaev is moving up to middleweight and he will face the No 1 welterweight in the world Kamaru Usman. So No 1 vs No 4 at middleweight, Usman vs Chimaev, I will see you in Abu Dhabi.'
Usman is entering the fight as the betting underdog, which isn't surprising given the short notice, the caliber of his opponent, and the fact that he recently suffered two losses to Leon Edwards in his last two fights.
However, the former dominant champion is eager to prove that he remains a formidable threat in both the 170-pound and now 185-pound weight classes.
It's worth noting that Alexander Volkanovski has stepped in to replace the injured Charles Oliveira in the main event, making UFC 294 shaping up to become one of the standout cards of the year. The organizers deserve significant credit for rescuing what could have been a disastrous situation.
Who's your money on! Usman or Chimaev? Let's hear your thoughts in the comment box below