Whittaker first won the interim title in July of previous year, elevated to the undisputed champion after newly crowned king Georges St-Pierre - who defeated then-champ Michael Bisping at UFC 217 in November - elected to vacate the throne after being diagnosed with ulcerative colitis.
Rockhold will remain in the card and in the main event, now facing late replacement Yoel Romero for the vacant UFC Interim Middleweight Championship.
ESPN reports that Whittaker will have to wait a little longer for the chance, as he's pulled himself from the fight against Luke Rockhold due to an undisclosed injury. "All the best to Yoel and Luke for their fight and I'll be waiting..."
UFC 221 is set to go down Saturday, February 10, from the Perth Arena in Perth, Australia.
"I'm beyond disappointed that I'm unable to defend the title in my home country of Australia". The victor will meet Whittaker in a title unification bout. The new main event at UFC 221 will still have championship gold resting in the balance as Rockhold vs. Romero will fight for an interim strap with the victor facing Whittaker in a unification bout later in 2018. The match will be for the interim title, and will no doubt follow up with a unification match as soon as Whittaker has healed.
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People who bought tickets for UFC 221 can receive a full refund.
33-year-old Rockhold, meanwhile, defeated Branch via TKO back in September. He defeated former two-division WSOF champion David Branch, the man Romero was originally scheduled to face at UFC on FOX 28 on February 24.
Whittaker was crowned interim champion last July with a win over Romero while Michael Bisping was injured and awaiting an eventual showdown with Georges St-Pierre in November.
Prior to that loss, Romero had won eight in a row, including six by knockout.