After losing his Octagon debut to Frank Mir at UFC 81 five years ago, Lesnar went on to win the promotion's Heavyweight title, defeating UFC Hall of Famer and former two-division champion Randy Couture at UFC 91, just four fights into his MMA career.
That was right before he sent Heath Herring into retirement after beating him down so bad at UFC 87 that "Crazy Horse" was never heard from again ... at least in the MMA world.
Lesnar went on to defend his title twice, getting brutal revenge on Mir at UFC 100 and then scoring a come-from-behind submission victory over Shane Carwin at UFC 116. He would eventually cough up the coveted crown to Cain Velasquez at UFC 121.
That's when his ongoing battle with diverticulitis and a subsequent surgery sidelined him for more than one year, forcing him to ponder MMA retirement. And according to his bitter rival, Mir, had Lesnar never gotten the life-threatening stomach disease, Lesnar's MMA career would have been drastically different.
His words (via New York Post):
"Honestly, I think the guy got so ill, he just couldn't do it anymore. He had to leave or his quality of life wouldn't have been normal. We didn't get to see the Brock I fought. Had he not been ill, things would have been different."
Lesnar returned to action at UFC 141 on Dec. 30, 2011, to challenge former DREAM and Strikeforce champion Alistair Overeem in what was the Dutch striker's promotional debut. However, things didn't go so smooth for Lesnar, who succumbed to the powerful strikes of "The Reem" in the very first round, handing him his second consecutive first-round technical knockout loss.
Lesnar announced his retirement from MMA during his post-fight speech and eventually found his way back to his old stomping grounds, World Wrestling Entertainment (WWE).
But, according to Mir, Lesnar could probably still be fighting and dominating MMA had he never gotten ill. And maybe Zuffa and UFC would be even more successful. After all, it was through Lesnar's blood, sweat and tears that the promotion is as successful as it is today.