UFC President Dana White says Diaz will fight B.J. Penn on the undercard at Mandalay Bay Events Center on Oct. 29, announcing the surprising matchup Thursday on Twitter.
Diaz was scheduled to fight St. Pierre for the UFC welterweight title, but White replaced him with Carlos Condit on Wednesday after Diaz refused to attend promotional news conferences in Toronto and Las Vegas this week.
White was furious with Diaz, saying the UFC probably would cut the Strikeforce welterweight champion from the promotion. A day later, White allowed Diaz to replace Condit, who had been scheduled to fight Penn at UFC 137.