Former UFC lightweight champion Rafael Dos Anjos will step in to fight either Conor McGregor Dustin Poirier should the other suffer an injury.
The Brazilian made the 156lb limit when he stepped on the scales on Friday and will be on hand to fight in the unlikely event either main event fighter is forced to withdraw.
Poirier and McGregor also successfully made weight, opting to also come in at 156lb, one pound heavier than 'championship weight'.
The rivals will come together for the ceremonial weigh-in later on Friday before touching gloves at the T-Mobile Arena 24 hours later.
McGregor was due to face Dos Anjos in 2016 but the then lightweight champion was forced to pull out injured and McGregor instead stepped up to welterweight and lost to Nate Diaz.
Earlier this week, McGregor reacted to reports that Dos Anjos was making weight by tweeting: "Put Dos Anjos in at the same time."
Dos Anjos replied by questioning why McGregor was tweeting in the early hours, saying: "Alcoholic [sic] hallucinosis from drinking cheap whiskey until 3:39am."
Having lost to Diaz, McGregor exacted his revenge five months later to level their rivalry.
Meanwhile, Dos Anjos was stopped by Eddie Alvarez as he surrendered his belt meaning there was no way back to a clash with McGregor.
The Dubliner instead stopped Alvarez in the second round of what McGregor has described as the best performance in UFC history.
But before he became the promotion's first 'champ-champ', he took another shot at Dos Anjos.
“Realistically I should be a two-weight UFC world champion,” he said before fighting Diaz for a second time.
“If I fought Rafael dos Anjos, if he didn’t p**** out of that last one and showed up, the shots I cracked Nate with in the first eight minutes would have KO’d Rafael dos Anjos stiff.
"That’s no question. I would be sitting here right now as a two-weight world champion, but I don’t dwell on that."
Dos Anjos hit back, vowing: “If you had fought me you wouldn’t be two-division champ, you would be dead. “Look what a out-of-shape Diaz did to you.”
The veteran has a record of 4-4 since losing his title in 2016 and returned to lightweight earlier year to defeat late-replacement opponent Paul Felder following a stint at 170lb.
That spell at welterweight included a defeat by Colby Covington and one to current champion Kamaru Usman.
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