UFC 324: Amanda Nunes fight postponed after Kayla Harrison undergoes neck surgery

The co-main event of UFC 324, featuring Kayla Harrison defending the women's bantamweight title against Amanda Nunes, has been postponed, CBS Sports has learned. Harrison underwent surgery on Tuesday to address herniated discs in her neck, according The Eagle Tribune, which first reported the postponement.

The Harrison-Nunes fight was considered by many to be the biggest bout in the history of women's mixed martial arts. The matchup will be moved to a later date that has yet to be determined. Sean O'Malley vs. Song Yadong will now serve as the co-main event in the bantamweight division. Plus, Rose Namajunas vs. Natalia Silva at women's flyweight was moved from the featured prelims to the main card.

The main card now reads as follows:

• Paddy Pimblett vs. Justin Gaethje, interim lightweight title
• Sean O'Malley vs. Song Yadong, bantamweights
• Waldo Cortes-Acosta vs. Derrick Lewis, heavyweights
• Natalia Silva vs. Rose Namajunas, women's flyweights
  
• Jean Silva vs. Arnold Allen, featherweights

Harrison, a two-time Olympic gold medalist in judo, won the bantamweight title with a second-round submission of Julianna Pena last June. The fight was Harrison's third in the Octagon after fighting the rest of her career in the PFL. Harrison has won all three of her UFC fights, with two wins by submission, clearing doubts many had that she would not be able to safely and consistently cut to bantamweight with her large frame. Harrison had fought at featherweight and, predominantly, lightweight during her PFL career.

Nunes is near unanimously considered the greatest female fighter in the history of the sport. She won the bantamweight championship at UFC 200, and after three successful defenses, moved to featherweight to win that title. Nunes moved back to 135 for four more successful title defenses before a shocking upset loss to Pena at UFC 269. Nunes won the rematch with Pena at UFC 277 and, after defending the title one final time against Irene Aldana at UFC 289, announced her retirement from the sport.

Nunes entered the Octagon after Harrison's win over Pena, signaling a return from retirement to set up a massive fight that will now have to wait.

In her career, Nunes holds 10 wins over women who have held UFC championships.

UFC 324 takes place Saturday, Jan. 24, from T-Mobile Arena in Las Vegas. The event is the first in the landmark media rights deal between UFC and Paramount. It will stream live on Paramount+ and is included with the price of a subscription as the deal also marked the end of pay-per-view UFC events.