In the co-main event Israel Adesanya looks to collect even more hype by finishing Anderson "The Spider" Silva.
It is understood Whittaker started complaining to his coaching staff late on Saturday evening of pain in his abdomen.
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.
UFC middleweight champ Robert Whittaker was forced to withdraw from Sunday's bout due to a hernia.
"Due to change in the main event, customers may request a full refund of their purchased tickets at the point of sale".
UFC officials have said the fight will remain over three rounds, and not go to five - as is the case for headline events.
Gastelum took to social media to let his fans know what was happening.
Follow live updates as three Kiwis fight at UFC 234.
On Twitter, Gastelum also wished Whittaker well in his recovery.
The 28-year-old was kept on the sidelines for the remainder of 2018 after a hand injury he sustained during the second Romero showdown. Prior to that, he knocked out former middleweight champion Michael Bisping in November 2017.
Whittaker was set to defend his title for the first time since becoming interim champion at UFC 213.