But Romero, who was a substitute for UFC middleweight champion Robert Whittaker, who pulled out of a scheduled bout with Rockhold due to illness, missed weight Friday, clocking in at 187.7 for a 185-pound fight.
In the co-main event of the evening American heavyweight Curtis Blaydes overcame an early scare to score the biggest victory of his MMA career as he outpointed Kiwi legend Mark "Super Samoan" Hunt after a gruelling three-round battle.
UFC 221 feels like Chaos Theory being tested to the max.
"The Soldier of God" adopted a measured pace against Rockhold, and after two competitive rounds, he found the opening he needed in the third stanza.
A fight between Whittaker and Romero now looks the logical next step as he looks to finally get his hands on the belt.
It's not that I had a problem with Luke Jumeau winning - I thought he would get the nod 29-28 against Daichi Abe - but when the scores read 29-27, 28-27 and 29-28, I would kind of like to know how those officials handed out the eights, nines and 10s to arrive at those totals.
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The third round saw more of the same, the duo duking it out at arms length before Romero suddenly unleashed a huge overhand left that caught Rockhold flush on the chin and sent him crashing to the canvas, before following it up with a vicious left uppercut as the referee jumped in to stop the fight. As Romero so often does, he came to life in the second round.
Rockhold, ranked No. 2, was expecting to put on a better showing than his last fight, when he earned a second-round TKO over David Branch at UFC Fight Night in Pittsburgh on September 16.
Romero did not appear at Saturday's post-fight press conference because he was transported to the hospital.
Though it might sound counterintuitive to award a title shot to an athlete that failed to make weight, Romero does have at least somewhat of an excuse, having taken the Rockhold fight on short notice. However despite the incredible victory, one has to wonder what's next for Yoel Romero.
The event featured the 12-fight bill, marking the debut of Ultimate Fighting Championship in Western Australia.
Why they should fight: Asker found out why UFC matchmakers have been hard-pressed to find willing opponents for Tai Tuivasa when he suffered a first-round TKO loss to the heavyweight prospect. He owns KO wins over the likes of Tim Kennedy, Lyoto Machida and Chris Weidman.
- Tyson Pedro def. Saparbek Safarov, light heavyweights, submission, first round (3:54).