The former UFC heavyweight champ was forced into early retirement after being diagnosed with diverticulitis, an intestinal disorder.
Since his exit from the UFC, Lesnar has returned to his old stomping grounds at World Wrestling Entertainment in search of a safer way to compete and still make a living.
It has been over a year since Lesnar officially announced his retirement from MMA, and Mir can't help but ponder the vast amount of untapped potential Lesnar left on the table.
"Honestly, I think the guy got so ill, he just couldn't do it anymore," Mir said in an interview with the New York Post. "He had to leave or his quality of life wouldn't have been normal."
Lesnar burst onto the scene in the UFC with only one professional fight to his name. In his debut, he was paired against Mir, a bona fide MMA legend and future UFC Hall of Famer. He dominated Mir in the early going before being submitted with a kneebar.
After the loss, Lesnar skyrocketed to the top of the heavyweight ranks by defeating Heath Herring and Randy Couture for the UFC title. The much anticipated rematch between Mir and Lesnar served as the main event for UFC 100.
The bout played out similarly as the original, but this time the outcome was different. There weren't any submissions to save Mir from a lopsided thrashing handed down by Lesnar's lunchbox-size fists.
Looking back, Mir is convinced the Lesnar that fought him that night wasn't the same fighter from then onward.
"We didn't get to see the Brock I fought. Had he not been ill, things would have been different," said Mir.
Not long after the fight, Lesnar was diagnosed with diverticulitis and forced to undergo major intestinal surgery. He returned a year later to defend his UFC title against Shane Carwin. After getting dominated early in the fight, Lesnar rallied in the second round with a submission win.
The amazing come-from-behind victory would be Lesnar's last in the UFC.
He went on to lose his next two bouts to Cain Velasquez and Alistair Overeem by first-round TKO stoppages. Before his final UFC bout with Overeem, Lesnar suffered a second case of diverticulitis, which required doctors to remove 12 inches of his colon.
It was an unfortunate way to see a promising career end. As a former NCAA Division I college wrestling champ and a freak athlete, Lesnar had the potential to do great things in MMA.
The heavyweight division is deeper than ever, but even now, Mir believes the old Lesnar would be among the best.
"The old Brock, I don't think I would have been too far off," Mir said.