Rivera's sisters, Lezlye and Kelly, remain hospitalized but in stable condition, the fighter's manager, Ali Abdelaziz, told ESPN.
Earlier Friday, in a Facebook post, Lezlye wrote that Rivera hadn't been sleeping, was not talking the way he normally would and completely had lost interest in MMA training, which is what "he lives to do." Lezlye said she flew to visit Rivera in Florida because of his bizarre behavior.
"Unfortunately his mental stage was worse then we could [have] imagined and before we could help him he completely broke," Lezlye wrote.
"His mentality has been completely corrupted into something unrecognizable."
According to an incident report from the Boynton Beach Police Department obtained by ESPN, Rivera, who was arrested Thursday, admitted that he tried to kill his two sisters by stabbing them with a knife in the early morning hours. Rivera said, according to the report, that killing his sisters was his purpose, told to him by "a higher power."
Rivera made his first appearance in court Friday morning and is now being held in a behavioral health facility after the court granted an emergency motion, according to online records. He was charged with two counts of attempted murder and is being held without bond.
Abdelaziz said Rivera was "very respectful" and a favorite of his Dominance MMA management team. He called Rivera's actions unusual, and like Rivera's sisters, he wants the fighter to get mental health assistance.
"His sisters support him 100 percent," Abdelaziz told ESPN. "They said, 'My brother is a good, beautiful human being and needs help.' They said it wasn't really him."
In the Facebook post, Lezlye wrote that she and her family "wish for [Rivera] to get the professional mental help he so desperately needs."
Rivera's mother, Susana Campos, told Mexican publication Milenio that her son "lost his memory and something evil took over him a few days ago. His mind was blocked. He wasn't himself."
Rivera, a 31-year-old Mexico native and Florida resident, debuted in the UFC last May and went 1-2 in 2020. He is a former Titan FC bantamweight champion.
In a statement Thursday, the UFC said it was investigating the "extremely troubling" situation and would not be offering Rivera a fight at this time.