In response to Robert Whittaker's injury, UFC president Dana White says the UFC's doctor have "never seen this type of injury in a young person." (2:12)

MELBOURNE, Australia -- An abdominal hernia has forced UFC middleweight champion Robert Whittaker out of his highly anticipated title defense against Kelvin Gastelum at UFC 234, just hours before doors opened at Rod Laver Arena.

UFC president Dana White told ESPN that Whittaker complained of pain in his abdomen late Saturday night (Australian time) and was taken to a local hospital. The Aussie champ was diagnosed with a hernia, underwent emergency surgery and is now recovering, according to White.

"This was a freak, freak injury," White told ESPN. "It's one of those situations where if it had popped out during the fight it could have been fatal for him."

The withdrawal is a huge blow to Whittaker, 28, who was forced to pull out of a title defense on home soil in February 2018. That withdrawal was also due to injury. Representatives for Whittaker did not immediately respond to requests for comment.

Gastelum wished Whittaker a speedy recovery.

With much sorrow I write that my fight for the middleweight title is off.

I'm sorry to everyone that was expecting this great fight, Family , friends and followers from around the world. — #OnAmission4Gold (@KelvinGastelum) February 9, 2019

God has a reason for doing things and I firmly believe his plans are much bigger and better than I can even imagine.

I'm so torn about this situation. A lifetime of pursuit to be able to reach this level and I hope none of these efforts will go in vain. #andnew #OnAmission4Gold — #OnAmission4Gold (@KelvinGastelum) February 9, 2019

I wish @robwhittakermma a speedy recovery champ! Get well soon! — #OnAmission4Gold (@KelvinGastelum) February 9, 2019

UFC has offered full refunds due to the cancellation of the bout.

A non-title middleweight bout between rising star Israel Adesanya (15-0) and Anderson Silva (34-8) will now headline UFC 234, which sold out in minutes when tickets first went on sale.

play 1:54 Should Robert Whittaker be stripped of his middleweight belt? Brett Okamoto, Chael Sonnen and Gilbert Melendez discuss if Robert Whittaker should be stripped of his belt after pulling out of UFC 234 due to an abdominal hernia.

Adesanya, who fights out of New Zealand, is one of the UFC's most promising prospects, taking on an all-time great in Silva.

Silva, 43, broke into tears during the UFC 234 official weigh-in. This is Silva's first appearance since February 2017. The UFC had already announced the winner of that fight would be next in line to challenge for the 185-pound title.