"I promised my wife and my kids if I won this fight, I would get a title shot, and that would be my last fight," Lesnar said. "But if I lost tonight ... you've been great."
Overeem will get the next shot at UFC heavyweight champion Junior Dos Santos, who watched from a seat near the octagon.
Overeem is a champion kick boxer who has fought in multiple promotions over the past decade, winning titles in Dream and Strikeforce with nearly unbeatable striking and size. He joined the UFC in September, finally presenting his formidable skills and intimidating physique to the sport's largest audience.
"My experience in UFC was, it's huge," Overeem said. "I think it's like 100 times bigger than Strikeforce. K-1 (kick boxing) is big, but this is a lot bigger. I was a little bit blown away, still am. I loved every second of it."
White might have given an immediate title shot to Overeem if the timing had been better, but Dos Santos only claimed Velasquez's belt in early November. Overeem welcomed a debut against Lesnar, even guaranteeing a knockout in the first two rounds.
"First or second round, I promised," Overeem said.
Lesnar hadn't fought since losing his heavyweight belt to Velasquez in October 2010, cancelling a bout against Dos Santos last June in Vancouver after another flare-up of diverticulitis. The former NCAA wrestling champion and fake WWE wrestler kept his unparalleled popularity during his recovery, and Lesnar used the time off to modify both his diet and his standup game, attempting to improve his biggest weakness.