Whittaker's withdrawal is a huge blow to UFC 234.
The lack of big fights has prevented Whittaker to find the amount of recognition that some of the other champions enjoy but that does not make him any less risky.
"He was taken to hospital and diagnosed with a hernia", Okamoto revealed.
The highly anticipated No. 1 contender match between rising New Zealand star Israel Adesanya (15-0) and Brazilian veteran Anderson Silva (34-8) will now serve as the main event on the Rod Laver Arena card.
The 28-year-old was scheduled to defend his belt against Yoel Romero at UFC 225, but the fight was changed to a non-title bout after Romero missed weight.
The loss of Whittaker will certainly be a massive disappointment for many fans with UFC 234 tickets.
Kiwi Israel Adesanya will now be headlining UFC 234 in Melbourne on Sunday against Anderson Silva.
Whittaker was supposed to be fighting for just the third time in a little over three years.
According to a report by ESPN, Whittaker suffered a hernia in his abdomen and could not be cleared to compete.
"Kelvin is the biggest hurdle in my way, and he's a true test", Whittaker said.
"I definitely think there should be some sort of feeding program for the UFC, like The Ultimate Fighter has been but I think The Ultimate Fighter has served its objective", Gastelum said ahead of his fight at UFC 234 (via MyMMANews).
"I'm the All Black look at me!"