UFC middleweight champion Robert Whittaker has withdrawn from a scheduled title defense against Luke Rockhold at UFC 221 next month due to an undisclosed injury.

Whittaker, 27, tried to remain on the event but was ruled medically unable to compete by physicians.

Rockhold (16-3) will now face Yoel Romero (12-2) for an interim 185-pound championship. UFC officials told ESPN that Whittaker will face the winner in a unification bout.

"I'm beyond disappointed that I'm unable to defend the title in my home country of Australia," Whittaker stated. "I hope the fans enjoy what should be a great night of fights at UFC 221, and I look forward to a recovery as soon as possible that will have me back in the Octagon to face the winner of the main event."

It's a major blow to the UFC's 221 pay-per-view event, which is scheduled to take place in Perth, Australia.

Whittaker was an obvious choice to headline the event, as he is the first Australia-based champion in UFC history. Customers who purchased tickets are eligible for a full refund.

Rockhold, who trains out of American Kickboxing Academy and Combat Club, is looking to recapture a UFC title. He won the 185-pound championship in December 2015, but surrendered it to Michael Bisping in his first attempted defense.

The 33-year-old former champion is coming off a TKO win over David Branch in September.

A former Cuban Olympic silver-medalist wrestler, Romero was scheduled to face Branch at a UFC Fight Night event on Feb. 24. He is actually coming off a decision loss to Whittaker for the interim title last July.

The switch is yet another strange turn in the UFC's middleweight division. In November, former welterweight champion Georges St-Pierre returned from a four-year absence to win the title against Bisping at UFC 217. St-Pierre opted to vacate the championship just 31 days later, citing health reasons.

Prior to that loss, Romero had won eight in a row, including six by knockout. Those wins included victories over Chris Weidman, Ronaldo Souza and Lyoto Machida.