It is understood Whittaker started complaining to his coaching staff late on Saturday evening of pain in his abdomen.
The bout will determine the next contender to the title, likely after Whittaker defends vs. Gastelum.
It is understood a devastated Whittaker will undergo surgery on Sunday.More news: Will Saudi Arabia be held to account for Khashoggi's murder?
Whittaker, 28, was forced to pull out of a title defense in February 2018 as a result of injury - representatives for Whittaker have not responded to requests for comment. Lando Vannata and Marcos Mariano's lightweight matchup will also be elevated to co-main status.
Injuries kept him from fighting Luke Rockhold at UFC 221 in Perth previous year.
Silva, 43, broke into tears during the UFC 234 official weigh-in.More news: 76ers finally trade Markelle Fultz, send him to Orlando Magic
UFC President Dana White confirmed the news to ESPN on Saturday.
With next to no time to find a replacement for Whittaker all the way out in Australia, UFC 234 turned to its old superstar in Silva to headline the event.
Those plans will now be delayed, given Gastelum is next in line for a title shot before the victor of the Adesanya-Silva showdown.More news: RBI Policy: MPC Cuts Repo Rate by 25 Basis Points to 6.25%