Gastelum was set to face UFC middleweight champion Robert Whittaker in the card's main event Sunday (Australian time).
A lightweight clash between Lando Vannata and Marcos Rosa Mariano will now move to the main card. "It wasn't a weight-cutting related issue or anything like that, it's just a very unfortunate hernia issue which may even require a procedure".
Whittaker was taken to hospital on Saturday morning, a source told ESPN's Ariel Helwani.
Rockets deal Ennis to the 76ers
The Houston Rockets have acquired guard Iman Shumpert in a three-way deal with the Cleveland Cavaliers and Sacramento Kings . Shumpert, 28, could become a key bench player for the Rockets as they pursue a deep run in the Western Conference playoffs.
In a later statement, UFC said Whittaker had undergone emergency surgery due to a "severe abdominal injury".
The event is going down on February 10, 2019 at Rod Laver Arena in Melbourne, Australia.
UFC legend Anderson Silva broke down in tears as he prepares to make his long-awaited return at UFC 234 in Australia.
Adesanya is undefeated from 15 fights in mixed martial arts, and won his first four fights in the UFC past year, including three victories that were adjudged performance of the night.