The event is going down on February 10, 2019 at Rod Laver Arena in Melbourne, Australia.
According to a report by ESPN, Whittaker suffered a hernia in his abdomen and could not be cleared to compete.
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.
Because his next fight against Yoel Romero was a non-title bout after Romero missed weight, Whittaker still has yet to officially defend his belt a year into his status as middleweight champ.
This is the second time Whittaker has been forced to withdraw from a scheduled title fight down under, after a range of injuries kept him from fighting Luke Rockhold at UFC 221 in Perth past year.
The new co-main is the lightweight matchup between Lando Vannata and Marcos Mariano.
UFC legend Anderson Silva broke down in tears as he prepares to make his long-awaited return at UFC 234 in Australia.
Abdelaziz said White told him that the UFC will "take care" of Gastelum.