McGregor 's put - up - or - shut - up moment is the main event of the UFC 's biggest card of the summer .
He is n't nearly the most accomplished mixed martial artist fighting in UFC 189 at the MGM Grand Garden Arena .
Although McGregor 's put - up - or - shut - up moment is the main event of the UFC 's biggest card of the summer , he is n't nearly the most accomplished mixed martial artist fighting in UFC 189 at the MGM Grand Garden Arena .