Middleweight Champion Robert Whittaker was forced to have 'emergency surgery' following his sudden withdrawal from UFC 234 on Saturday.
Whittaker, 28, has had a wretched run trying to defend his title on home soil, having also missed a clash against Luke Rockhold at UFC 221 last February due to serious illness.
He is a heavy favorite going up against the 43-year old Silva, who has not fought in two years, and a victory should catapult the entertaining Style Bender into the main event scene.More news: Mavs trade Harrison Barnes mid-game, get bashed by LeBron
More news: Saudi crown prince had threatened to use 'bullet' on Khashoggi - NY Times
More news: Measles Outbreak Sends Vaccine Demand Soaring, Even Among The Hesitant
UFC president Dana White stated that Whittaker began complaining of issues after weighing in Friday night and was taken to a local hospital.
Fighting out of Sydney, Whittaker is the UFC's first titleholder from Australia.
It is unclear how long Whittaker will be inactive, but Gastelum's manager Ali Abdelaziz has said that he would like his fighter to face the victor of Adesanya vs Silva. They were initially slated to be the co-main event in Melbourne.
Whittaker last fought Yoel Romero in a five-round war at UFC 225 in Chicago.
On Saturday, Whittaker and Gastelum both made weight, with "The Reaper" coming in at 185-pounds and the American challenger at 184-pounds.