UFC boss Dana White posted a picture on Twitter of the two men above the caption '200' upon announcing the headline bout late on Wednesday.
White later told ESPN SportsCenter he and UFC co-owner Lorenzo Fertitta had encouraged McGregor (19-3) not to fight at 170 pounds, 25 more than the trash-talking 27-year-old's usual featherweight limit.
"He was obsessed -- obsessed with fighting Nate Diaz again," White said.
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"Obviously, Lorenzo and I tried to argue with him and said, 'Let's go back to 145 and defend your title. Or if you really want the Diaz fight that bad, do it at 155.' He wants the fight at 170.
"Even his coach tried to get him to get off the rematch and off the 170-pound fight... but it's what he wanted."
McGregor had hurt Diaz early in their first bout in Las Vegas but could not capitalise and the bigger man landed some punishing blows himself before his jiu-jitsu talents won him the fight.
White said Jose Aldo and Frankie Edgar would also square off on the UFC 200 card, with the winner then taking on McGregor for the featherweight belt regardless of the Irishman's result against Diaz.
(Writing by Patrick Johnston in Singapore; Editing by John O'Brien)
