After the first-round bell, McGregor (22-6) was unable to continue and Poirier (28-6) registered his second consecutive win over the Irish fighter.
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According to Poirier, McGregor's leg broke in the early stages of the fight.“I felt something, for sure. He fractured it on one of the kicks at the beginning of the fight, and then he broke it," Poirier said. The American MMA fighter will get the UFC’s next lightweight title shot.
McGregor and Poirier have been prime rivals since 2014, when the former won at UFC 178 with a first-round KO. However, Poirier tasted success in January 2021's showdown and again on Saturday.
Poirier dodged well, attacked well in the first round before getting McGregor on the ground. A series of punches and elbows was followed by the knockout.
“This is not over!” McGregor shouted as he was carried from the cage.
Former US President Donald Trump also made a brief appearance at the T-Mobile Arena. As the Republican leader arrived, fans cheered and booed.
Trump entered through the fighter tunnel before the welterweight bout between Gilbert Burns and Stephen Thompson. As he walked, "U-S-A" chants broke out.
Former MMA fighter, Khabib Nurmagomedov took on to Twitter to say, "Good always defeats evil. Very happy for @DustinPoirier I hope you will get the belt end of the year."
Earlier, UFC president Dana White had said that Saturday's winner will fight for the UFC lightweight title next against champion Charles Oliveira. However, after the anti-climax finish, there is speculation about a fourth bout.