The news, in which Whittaker reportedly suffered a hernia, initially came via ESPN's Brett Okamoto.
The loss of Whittaker, who is on a nine-fight win streak, will certainly be a massive disappointment for many fans with UFC 234 tickets.
Ironically, injury also forced Whittaker out of his last scheduled title defence in Australia, a UFC 221 clash with Luke Rockhold in Perth. The issue is not believed to be related to his weight cut.
It means the no less anticipated co-main event between Israel "The Last Stylebender" Adesanya and Brazilian legend Anderson Silva will top proceedings Down Under. That would have been Whittaker's first bout since being named middleweight champion following Georges St Pierre's surrendering of the title. A lightweight clash between Lando Vannata and Marcos Rosa Mariano that was originally scheduled for the Fight Pass prelims has been moved to the pay-per-view lineup.
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Whittaker was taken to hospital on Saturday morning, a source told ESPN's Ariel Helwani.
Those plans will now be delayed, given Gastelum is next in line for a title shot before the victor of the Adesanya-Silva showdown.
UFC officials have said the fight will remain over three rounds, and not go to five - as is the case for headline events. Now, Adesanya gets the biggest fight of his career thanks to a UFC card being suddenly Whittaker-less again.