According to a report by ESPN, Whittaker suffered a hernia in his abdomen and could not be cleared to compete.
"He was taken to hospital and diagnosed with a hernia", ESPN reporter Brett Okamoto revealed. He will have surgery to resolve the issue immediately.
UFC's rising star Israel Adesanya (15-0) and Anderson Silva (34-8) will now headline UFC 234, with the victor promised a shot at the title later in the year although now with Whittaker off the card the potential date is uncertain.
Australia's middleweight champion Whittaker (20-4) was scheduled to defend his belt on home soil at Rod Laver Arena against Kelvin Gastelum (15-3, 1nc).
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Adesanya, a New Zealander who UFC called its breakout star of 2018 in its release, made his UFC debut only a year ago in the preliminary card of UFC 221, the same event that Whittaker had to pull out of.
Per Okamoto, the hernia was not related to Whittaker's weight cut. Adesanaya vs. Silva will remain a three-round contest.
The new co-main is the lightweight matchup between Lando Vannata and Marcos Mariano.
Those plans will now be delayed, given Gastelum is next in line for a title shot before the victor of the Adesanya-Silva showdown.
There are a total of five fights on the main card.