UFC 234 has lost its headliner just hours before the show is set to begin.
For all the good luck that the UFC had in keeping fights scheduled through January, February is not starting off the same way.
The news, in which Whittaker reportedly suffered a hernia, initially came via ESPN's Brett Okamoto. In the co-main event Israel Adesanya looks to collect even more hype by finishing Anderson "The Spider" Silva.
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The forward was then included in the squad announced at 1900 GMT on Tuesday. Bookmaker companies consider Barça the favorites in this clash.
ESPN first reported the news and said Whittaker's withdrawal was not a result of weight cutting issues. The event was scheduled to take place in Melbourne, Australia, due to the 28-year-old's status as the first Australian to hold a UFC belt.
It's the second Australian headliner that Whittaker has been forced to pull out of.
Gastelum won a state wrestling championship in high school, although Whittaker's amateur achievements in freestyle wrestling, as well as his proven ability to stop any-and-every takedown, make it unlikely that this fight spends much time on the ground.
Whittaker won the interim middleweight title in at UFC 213 in July 2017 with a unanimous decision against Romero. Whittaker has yet to defend his belt after Romero missed weight for their rematch.