Expected to be one of the biggest tests of his career, Adesanya passed with flying colours when he stopped Costa in the second round. It was a dominant performance, handing Costa the first loss of his career.
However, some viewers noticed Adesanya had a swollen right pectoral muscle and immediately set the internet alight with accusations that the Kiwi champion had been doping.
One of the more well-known triggers for gynecomastia, the medical term for swelling in male breast tissue, is the use of anabolic steroids or androgens, but it can also be caused by a wide variety of factors, including drugs and alcohol, various medications, and even herbal products.
A couple of weeks after the fight, Adesanya said he had been examined by UFC doctors and had his hormone levels checked, before noting marijuana use could have contributed.
"They talk about titty-gate, but they're just looking to find excuses to take away my greatness," Adesanya said on Thursday. "I understand this is what people are supposed to do so I let them. I would give $3million to anyone who could ever have concrete evidence that I even know what I'm doing with steroids. I promise you, $3million. Pull up.
"It's easy to talk but really, it got to me a little bit after the Costa fight. I had a spectacular performance, so it was like, 'how the f*** are you trying to take that away with accusations based on nothing?'
"So yeah, pull up. $3million. I know you don't have that in the bank, but I do."
This weekend, Adesanya is set for the latest challenge to his standing as the world's best middleweight with knockout artist Jared Cannonier stepping up to the mark.
Cannonier has been formidable in the middleweight division since dropping down from light heavyweight, with five wins in six bouts at 84kg – four of those by knockout.
It's not the first time Adesanya has come into a bout hearing of the power an opponent possesses – it was a similar storyline when he met Costa as well as Yoel Romero, both of whom Adesanya beat without sustaining much damage at all.
"They keep talking about power, power, power. Have you ever heard that one before?"
The bout headlines UFC 276 – the showpiece of the UFC's International Fight Week celebrations – with Australian featherweight champion Alexander Volkanovski in the co-main event, Kiwi lightweight Brad Riddell in the featured preliminary bout, while Australian bantamweight Jessica-Rose Clark opens proceedings in what shapes up as the biggest UFC event ever for the Oceania region.
UFC 276
T-Mobile Arena, Las Vegas. Sunday.
Early prelims (UFC Fight Pass) - 10am
Prelims - ft. Brad Riddell (ESPN, Prime, Sky Sport Now, UFC Fight Pass) - 12pm
Main card (pay-per-view) - ft. Israel Adesanya; Alexander Volkanovski (Sky Arena, Spark Sport, Sky Sport Now, UFC Fight Pass) - 2pm
