Sunday's scheduled clash with American Kelvin Gastelumn at a sold-out Rod Laver Arena in Melbourne was called off in the early hours after Whittaker experienced issues on Saturday evening and subsequently underwent surgery.

Sixth-ranked middleweight Israel Adesanya scored a unanimous three-round points win over former champion Anderson Silva in a bout that subsequently elevated to the status of sole main event.

New Zealander Adesanya is slated to be the next contender for the title, after the Whittaker-Gastelum bout is rescheduled.

UFC boss Dana White said Whittaker 'had suffered a less than one per cent, freak, injury.

"What they told me was that his colon and intestines had popped out,' White said in the post-show press conference.

"They don't think it was anything he did yesterday, they think he's had it for a while and it's been popping in and out.

"The scary part is that if he took a shot to the stomach he could have died.

"He could rupture the colon and become septic and that's not good.

"Everything happens for a reason. It's great the way this worked out, to find out this kid had this problem and was unhealthy and we were able to get it fixed and they think he'll be out for four to six weeks."

Whittaker first reported issues on Saturday evening, but White said the champion didn't want to go to hospital until Sunday morning.

"He was actually going to push through it and fight, thank God he didn't," White said.

It continued a terrible run of bad luck and health issues for 28-year-old Whittaker since he won the interim title in 2017.

Illness and injury forced him out of a title defence in Perth 12 months ago and he suffered a broken hand in his last bout with Yoel Romero in June 2018.

Whittaker's manager Titus Day said the fighter reported feeling bloated and tired around 6pm on Saturday.

"Then about 10 o'clock we got a call from (Whittaker's wife) Sofia saying he's got some bad stomach cramps and he's in quite a lot of pain and can't sleep and can't go to the bathroom," Day told AAP.

"We said we'll call the UFC doctors and shortly after that he started vomiting so everyone was alerted.

"Rob is absolutely shattered for everyone else.

'"This is Rob's huge special moment in life, it's just been shattered, so there will be a lot of pieces to be picked up and we'll do it."

A disappointed but playful Gastelum borrowed the belt of flyweight champion Henry Cerjudo, who attended the show, declaring he was the champion as Whitaker had effectively forfeited the fight.

"I was devastated when they told me the news, I wish Rob all the best, a speedy recovery and hope he recovers a hundred per cent," Gastelum told ESPN.

There was mixed fortunes for Australians Jim Crute and Nadia Kassem, each of who was making their second UFC appearance after a first-up win.

Victorian light heavyweight Crute got a first-round stoppage over American Sam Alvey, who clearly felt the fight was stopped prematurely.

Kassem suffered a second round submission loss to world rated American flyweight Montana De La Rosa, who had the Sydneysider on her back for much of the bout.

Australian Associated Press