UFC 234 has in an instant turned from a beastly, salivating pay-per-view card into an event that the world's UFC commentators are poking fun at.

The Aussie blockbuster has been turned on its head by the shock hospitalisation and withdrawal of middleweight champion Robert Whittaker after he suffered a abdominal hernia on Sunday evening.

Whittaker has been forced to abandon his title defence due to a severe abdominal injury requiring emergency surgery. The middleweight had hoped a raucous home crowd in Melbourne would help him keep his world title against former bounty hunter American Kelvin Gastelum in the Octagon on Sunday.

However plans for Whittaker's first official title defence were thrown into disarray late on Saturday night when the 29-year-old complained of abdominal pain before being diagnosed with a hernia in hospital.

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His withdrawal was made official on Sunday morning.

UFC TV commentator Megan Olivi reported on Sunday that Whittaker's surgery was successful and he has an early forecast recovery of 4-8 weeks before he is able to resume his full-time training load.

The three-round middleweight match between rising New Zealand sensation Israel Adesanya (15-0) and Brazilian legend Anderson Silva (34-8, 1nc) will now serve as the main event at Rod Laver Arena.

Elevated to the co-main is the lightweight match-up between Albuquerque action hero Lando Vannata and surging Brazilian Marcos Mariano.

The wildly popular UFC 234 card sold out in just eight minutes. It's now the laughing stock of the UFC.

The fact that the main event will remain a three-round bout between Silva and Adesanya has resulted in some UFC critics labelling the Melbourne Card "the worst in UFC history".

The complication of Whittaker's withdrawal has proven to be a double edged sword, with the Adesanya-Silva fight cheapened by the fact that the winner will now have to wait in line for Gastelum to get his shot at Whittaker before either of them are given a title shot — as promised by Dana White during Friday's press conference, before Whittaker's hernia threw the whole division into disarray.

Gastelum and a host of other UFC fighters were left devastated by Whittaker's emergency.

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Whittaker (20-4) is on a nine-fight winning streak and hasn't lost in five years although he was on the comeback after breaking his hand in his epic showdown with Yoel Romero last June.

Gastelum (15-3 plus one no contest) had predicted the fight would finish in a first round knock-out.

"He might — a lot of people have said that," Whittaker said in Melbourne on Friday.