Rivera confirmed the shakeup on Monday's edition of The MMA Hour, saying that he got the call at 11:30 a.m. ET.
"I thought it was a joke," he said. "I couldn't believe it."
Rivera's shock came due to the history between the men, as this is the third proposed fight between the duo that has had to be scuttled due to injury.
The first bout was supposed to happen at UFC 118 in August 2010, but an arm injury forced Rivera to withdraw. Then, it was rescheduled as the co-main event of UFC 122. On the day of the fight, Sakara came down with flu-like symptoms and could not compete, scrapping the bout.
This time, it's a knee injury that has scratched the seemingly cursed matchup.
Rivera said that he believes the UFC will match him with Constantinos Philippou, who made his organizational debut on short-notice at UFC 128, losing a decision to Nick Catone. Philippou is already scheduled to be on UFC 133, against Rafael Natal.
Rivera said that he believes Philippou would make for a good style matchup and exciting fight, even if he will have to do some last-minute homework to prepare for an unexpected foe.
"I don't need the excitement, to be honest," he said. "It's coming and I'm sure he'll be pumped. He's going from the undercard to main card, fighting a bigger name. He'll come ready to fight. But this keeps me on my toes. I just want to have a good fight and give the fans something to cheer about and get excited about."
UPDATE: The UFC has since confirmed Philippou as Rivera's new opponent.
In his last fight, Rivera suffered a second-round TKO at the hands of Michael Bisping. Afterward, Rivera seemed to entertain the possibility of retirement, but he said on Monday that he would take a cautious approach to his future
"Honestly what I was saying is I live day-to-day," he said. "If I were to get smashed I'd contemplate it. If I smash him, I'll take the next fight and keep rolling. I'm not going to make rash decisions ahead of time. We'll see how it goes."