Whittaker, who successfully weighed in on Friday, was experiencing pain in his abdomen late last evening and was immediately taken to the hospital where he was diagnosed.
A devastated Whittaker was set to undergo surgery on Sunday after the hernia left him vomiting the night before.
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.
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UFC 234 takes place Saturday at Rod Laver Arena in Melbourne, Australia.
For all the good luck that the UFC had in keeping fights scheduled through January, February is not starting off the same way. The bout will determine the next contender to the title, likely after Whittaker defends vs. Gastelum.
Elevated to the co-main is the lightweight matchup between Albuquerque action hero Lando Vannata and surging Brazilian Marcos Mariano. We still will be getting what should be a stellar middleweight showcase in the main event with a prominent name attached - it just won't be contested for the 185-pound title.
Earlier this week, White had said the winners of the Whittaker-Gastelum and Adesanya-Silva fights would meet later this year.