Demetrious (Mighty Mouse) Johnson will defend his flyweight title against Japan's Kyoji Horiguchi, ranked eighth among 125-pound contenders, in the co-main event of UFC 186.
The Johnson-Horiguchi bout replaces the original co-main event featuring welterweight contenders Rory MacDonald and Hector Lombard.
While the UFC offered no reason for pulling the MacDonald-Lombard fight, the Nevada State Athletic Commission confirmed later in the day that Lombard had tested positive for an anabolic steroid at UFC 182 on Jan. 3.
The 37-year-old Lombard, who competed in judo for Cuba at the 2000 Olympics, tweeted earlier in the day that he learned of the fight cancellation via a reporter's text.
"I don't have a clue at this point, but I trust the UFC made a decision for a good reason!" the fighter tweeted.
It's another black eye for the UFC. Former middleweight champion Anderson Silva also tested positive for anabolic steroids at an out-of-competition test before UFC 183. Silva has denied cheating.
Lombard (35-4-1 with one no contest) is ranked fifth among welterweight contenders. He won a unanimous decision over defeated Josh Burkman at UFC 182.
The UFC said it is looking for another fight for MacDonald, a top Canadian fighter who has had bad luck of late.
MacDonald, ranked No. 2 among 170-pound contenders, had hoped to fight for the title next. UFC president Dana White said as much before then-champion Johny Hendricks faced (Ruthless) Robbie Lawler at UFC 181 in December. Hendricks had beaten Lawler at UFC 171 last March to claim the title vacated by Montreal's Georges St-Pierre.
Instead of having MacDonald (18-2) face the winner, the UFC opted to stage a third Hendricks-Lawler bout.
With Lawler opting not to fight until the summer, Hendricks has been matched again Matt (The Immortal) Brown at UFC 185 on March 14 in Dallas.
The 25-year-old MacDonald, a native of Kelowna, B.C., who fights out of Montreal, has won eight of his last nine bouts.
Johnson (21-2-1) has defended his flyweight title five times since winning a split decision over Joseph Benavidez at UFC 152 in September 2012 to become the first 125-pound champion.
Horiguchi (15-1) has won all four of his UFC fights.
Bantamweight title-holder T.J. Dillashaw meets Brazil's Renan Barao in the main event at the Bell Centre.
The UFC also added a lightweight bout to the Montreal show with Chad (The Disciple) Laprise (10-0) facing Bryan Barberena (10-2).
Laprise, a native of Windsor, Ont., who trains in Montreal, won "The Ultimate Fighter Nations" welterweight crown.
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