He later said Whittaker would be undergoing surgery on Sunday for the problem. That is when tests revealed the champ would need surgery.
In the main event of the UFC 234 Aussie middleweight champion Robert Whitaker defends his title against USA rival Kelvin Gastelum.
This is the second time in 12 months that Whittaker has had to withdraw from a title defence on home soil, after injury forced him out of the card at UFC 221 in Perth last February. The new feature attraction will be the old co-main event as Anderson Silva and Israel Adesanya will close the show in Melbourne.
Extra weight was added to the Adesanya-Silva fight when UFC president Dana White announced on Friday that it would be for the No.1 middleweight contender spot.
"The new main event, which airs live on Pay-Per-View, will be a three-round middleweight bout between Anderson Silva and Israel Adesanya".
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Gastelum took to social media to let his fans know what was happening.
Gastelum, of California, is riding a two-fight win streak, including a split-decision victory over Ronaldo "Jacare" Souza.
On Twitter, Gastelum also wished Whittaker well in his recovery.
According to UFC officials (via MMAJunkie) Whittaker came down with a "severe abdominal injury" that will keep the middleweight champion from defending his title tonight.
In Australia working for ESPN, Sonnen is now contracted to Bellator, but said the promotion had allowed him to step in if necessary.