The 29-year-old American was to defend his 145-pound title on April 25 in a rematch with Brazilian Renan Barao. But the champion fractured a rib in training Monday.
"Just a freak accident during wrestling practice," Dillashaw said via social media 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."
There was no immediate word on when Dillashaw might be able to return to action.
The new main event at the Bell Centre will be a flyweight title fight between champion Demetrious (Mighty Mouse) Johnson and Kyoji Horiguchi, ranked eighth among 125-pounder contenders. A light-heavyweight bout between former title-holder Quinton (Rampage) Jackson and No. 13 Fabio Maldinado has been promoted to co-main event.
Montreal, once the bedrock of Canadian UFC shows, has been taking it on the chin recently.
A show planned for late 2014 was called off due to trouble finding a main event. Canadian welterweight contender Rory Macdonald, a native of Kelowna, B.C., who fights out of Montreal, was set to meet Hector Lombard in the original UFC 186 co-main event. That matchup was pulled after Lombard flunked a drug test and replaced by Johnson-Horiguchi.
MacDonald is now meeting 170-pound champion (Ruthless) Robbie Lawler on July 11 at UFC 189 in Las Vegas.
Dillashaw upset Barao via fifth-round KO last May to win the title at UFC 173. The rematch was slated for UFC 177 in August but Barao (35-2 with one no contest) took ill during his weight cut and was replaced by Joe Soto. Dillashaw won by a fifth-round stoppage.
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