Gonzaga, who was scheduled to face Roy Nelson (16-7) on the five-fight heavyweight main card, has been replaced with Dave "Pee Wee" Herman (21-3). Both are coming off of losses, so the matchup will be a must-win fight for both.
Herman's record includes 21 victories with 15 of those having come by knockout. Nelson is 1-3 in his last four.
In a lightweight matchup with Edson Barboza (10-0), Jamie Varner (19-6-1) returns to the UFC to replace Dunham on the preliminary card. After a four-fight skid without a win, Varner picked up three victories in his last four bouts, including a submission win over Drew Fickett.
The first major hit to the card came when Alistair Overeem was removed from his fight with UFC heavyweight champion Junior dos Santos due to a pre-fight drug test.
Overeem was replaced with former champion Frank Mir, who is coming off of three wins, including those against Mirko Cro Cop, Antonio Rodrigo Nogueira and Nelson.
In the co-main event, Cain Velasquez and Antonio Silva will square off in fight between two fighters looking to prove they still belong at the top of the division. Velasquez was knocked out by dos Santos in his last fight, while Silva was knocked out by Daniel Cormier.
Other main card bouts include Shane del Rosario vs. Stipe Miocic and Mark Hunt vs. Stefan Struve. Hunt vs. Struve is the only main card fight to remain the same since the card was announced.
The card will take place May 26 on pay-per-view and be broadcast live from the MGM Grand Garden Arena in Las Vegas.