In a later statement, UFC said Whittaker had undergone emergency surgery due to a "severe abdominal injury".
After tests, he was rushed into surgery on Sunday morning.
Whittaker's withdrawal has thrown a spanner in the works for UFC president Dana White, who was banking on the winners of the main and co-main events to square off later this year.
In the main event of the UFC 234 Aussie middleweight champion Robert Whitaker defends his title against United States rival Kelvin Gastelum.
Sixers trade Markelle Fultz to Orlando Magic
He had from the same injury and was forced to sit for much of his rookie season past year , appearing in just 14 games. Simmons' deal is much more tenable. "So, we believe that, at 20 years old, the sky's the limit for him".
Gastelum took to social media to let his fans know what was happening.
Silva, 43, broke into tears during the UFC 234 official weigh-in.
Bezos's stunning blackmail charge intensifies proxy war with Trump
That prompted Bezos to launch an investigation into the Enquirer , and how it was able to obtain such intimate material. Bezos , the Board has convened and determined that it should promptly and thoroughly investigated the claims.
Gastelum wished Whittaker a speedy recovery.
The 28-year-old was kept on the sidelines for the remainder of 2018 after a hand injury he sustained during the second Romero showdown.
The cancellation of the bout poses the question of whether or not Gastelum will get the first shot at Whittaker's belt when he returns from injury, with the victor of the fight between Adesanya and Silva guaranteed a title shot.
In Australia working for ESPN, Sonnen is now contracted to Bellator, but said the promotion had allowed him to step in if necessary.
Thailand says Australia to blame for arrest of refugee Bahraini footballer
The FFA has taken the decision to scrap a preparatory camp in Thailand's capital in protest at Al-Araibi's ongoing detainment. We'll just consider the case based on the law and the need to maintain neutrality.