Whittaker was set to defend his title in his native country of Australia at UFC's first pay-per-view event of 2019.
Robert Whittaker was ruled out from fighting against Kelvin Gastelum at UFC 234 on Saturday due to an abdominal hernia, UFC president Dana White reportedly told ESPN's Brett Okamoto.
According to Okamoto, Whittaker complained of abdominal pain the night before the fight and it was determined that he would need surgery. Whittaker underwent the surgery on Saturday.
Whittaker, who fights out of Sydney, Australia, was set defend his undisputed middleweight title at Rod Laver Arena in Melbourne, Australia. Whittaker is currently 20–4 and has won each of his last nine fights.
Whittaker-Gastelum was meant to serve as a co-main event for Saturday's UFC event. Former middleweight champion Anderson Silva will face Israel Adesanya, who's undefeated, as the lone headline bout following Whittaker's departure.