"He was taken to hospital and diagnosed with a hernia", Okamoto revealed.
With Whittaker out, Anderson Silva's long-awaited return to the octagon against rising star Israel Adesanya has been elevated to the UFC 234 main event. Gastelum's split-decision victory over the former fighter at UFC 224 in May 2018 earned him this title shot.
The event was scheduled to take place in Melbourne, with the 28-year-old the first Australian to hold a UFC belt.
The loss of Whittaker will certainly be a massive disappointment for many fans with UFC 234 tickets.
Whittaker was supposed to be fighting for just the third time in a little over three years.
A devastated Whittaker was set to undergo surgery on Sunday after the hernia left him vomiting the night before.
Whittaker's opponent - Kelvin Gastelum - has also impressed everyone with his performances off late and his last two wins came against big names like Rondaldo Souza and Michael Bisping.
Because his next fight against Yael 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. "I look forward to learning something from it and growing".