First was current UFC Women's Bantamweight champion Holly Holm, vanquisher of previously reigning champ Ronda Rousey, submitting due to a Meisha Tate rear-naked choke that knocked her out in the middle of the fifth and final round.
The dual upsets stunned a sellout crowd and one of the biggest pay-per-view audiences in the history of mixed martial arts.
Tate (18-5, 5-2 UFC) has now won five fights in a row since losing to Ronda Rousey at UFC 168.
Diaz and Tate had plans of their own. "If they want to make that rematch happen, that's how I see it going down, because on Saturday, I'm going to go in there, and I'm confident I'm going to win the belt".
McGregor (19-3, 7-1 UFC), who bumped up to 170 pounds for the fight, was in control of the fight after the first round, cutting Diaz near his right eye.
Originally, the scheduled fight was between McGregor and lightweight champion Rafael Dos Anjos in a champion-against-champion super fight, but the latter got injured three weeks prior to this event, forcing the UFC to find a replacement and Diaz answered the call and got the job done emphatically. When McGregor tried a takedown, Diaz quickly forced him to tap out on the ground.
"I felt I took him in the first round, but I was inefficient with my energy", McGregor, who usually fights at 145 pounds, said after the fight. "If I would have had a full camp, I don't think I would have been touched".
Holm was up and moving around moments afterward, and Tate praised her fallen opponent.
Police officer, suspect dead after North Texas park shootout
The other officer was Michael Williamson, who was killed by a drunken driver in 1982, according to the city website. Officer Guidon and her partner were also allegedly ambushed by the suspect, Army Staff Sergeant Ronald Hamilton.
Remaining on the front foot, the SBG fighter seemed to tighten his grip on proceedings with a solid start to the second stanza, landing a crisp left shot to the body and a couple more to Diaz's bloodied face.
She paid tribute to the outgoing champion who had refused to "tap out" hanging on until she physically passed out, and added: "I had to be patient". She's tougher than nails. Tonight, I made some mistakes.
The main event was the greatest moment in the career of Diaz, a pugnacious veteran from a notorious fighting family in Stockton.
McGregor, who still holds his featherweight title as this bout took place two divisions up at welterweight, said as much in defeat.
McGregor selling the fight is possible and his performance Saturday night against Diaz supports that. "I'm humble in victory and in defeat".
"This is part of the game". "I took a chance to move up in weight and it didn't work".
Mr. Diaz started slow as the Irishman took the fight to him early, connecting with some solid blows that stymied Mr. Diaz's attack. I'll fix those mistakes and come back stronger. "Hope to come back better next time", Mr. McGregor said.