This is the second time that Whittaker was scheduled to defend the UFC middleweight title in Australia, only for injury to prevent such a thing from happening. The issue is not believed to be related to his weight cut. That would have been Whittaker's first bout since being named middleweight champion following Georges St Pierre's surrendering of the title.
Elevated to the co-main is the lightweight matchup between Albuquerque action hero Lando Vannata and surging Brazilian Marcos Mariano.
UFC president Dana White says the 29-year-old Whittaker, who has a record of 20-4, became ill Saturday night and complained of pain in his abdomen before being diagnosed at a hospital with a hernia.
It is understood a devastated Whittaker will undergo surgery on Sunday.
UFC officials have said the fight will remain over three rounds, and not go to five - as is the case for headline events.
UFC 234 takes place at Rod Laver Arena in Melbourne, Australia.
Those plans will now be delayed, given Gastelum is next in line for a title shot before the victor of the Adesanya-Silva showdown.
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.