According to a report by ESPN, Whittaker suffered a hernia in his abdomen and could not be cleared to compete.
A UFC spokesperson said the issue was not related to his weight cut.
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. That would have been Whittaker's first bout since being named middleweight champion following Georges St Pierre's surrendering of the title.
UFC officials have said the fight will remain over three rounds, and not go to five - as is the case for headline events.
Oil falls below $62 on USA inventories, fading Venezuela concerns
Maduro, who retains control over the state, denounces Guaido as a US puppet who is seeking to foment a coup against him. Oil fell on Tuesday after a survey showed euro zone business expansion almost stalled in January.
Elevated to the co-main is the lightweight matchup between American Lando Vannata and Brazilian Marcos Mariano.
Silva, who is officially 1-4-1 in his last six fights, is returning from a two-year hiatus from fighting due to failed drug tests.
UFC legend Anderson Silva broke down in tears as he prepares to make his long-awaited return at UFC 234 in Australia.
UFC 234 has been plunged into turmoil by Robert Whittaker's injury exit on Saturday night. Now, Adesanya gets the biggest fight of his career thanks to a UFC card being suddenly Whittaker-less again.