Taking its place at the top of the bill will be the former co-main event between Kiwi phenom Israel Adesanya and Brazilian legend Anderson Silva.
The cancellation of the bout poses the question of whether or not Gastelum will get the first shot at Whittaker's belt when he returns from injury, with the winner of the fight between Adesanya and Silva guaranteed a title shot.
Elevated to the co-main is the lightweight matchup between Albuquerque action hero Lando Vannata and surging Brazilian Marcos Mariano.
Despite being elevated to the main event, Adesanya's bout will remain a three-round fight.
This is the second time in 12 months that Whittaker has had to withdraw from a title defence on home soil, after injury forced him out of the card at UFC 221 in Perth last February.
ESPN first reported the news and said Whittaker's withdrawal was not a result of weight cutting issues.
"He was taken to hospital and diagnosed with a hernia," ESPN reporter Brett Okamoto revealed. "It wasn't a weight-cutting related issue or anything like that, it's just a very unfortunate hernia issue which may even require a procedure."
UFC President Dana White later confirmed the 28-year-old would be undertaking surgery today.
The Sydney-based star has experienced a horror run of injuries and illness throughout his career, including a gruesome abscess infection which forced him to pull out of UFC 221 in 2018.
