The 29-year-old was scheduled to defend his bantamweight title against Renan Barao in Montreal, but a "freak" rib injury suffered in training on Monday has seen the bout cancelled.
"Just a freak accident during wrestling practice," Dillashaw said on social media on Tuesday. "This is heart-breaking to me because I was looking forward to this fight and never want to be a fighter having to pull out of a fight. I will heal up quick and come back as soon as I can. Thx for the support."
UFC chairman Dana White confirmed the news during an interview on KRRL-FM 92.3, and said that there was no timetable for Dillashaw's return.
It is expected that the flyweight title fight between champion Demetrious Johnson and Kyoji Horiguchi will now become the main event in Montreal.