According to physician Matt Pitt at Sherdog.com, so was his very life:
Diverticulitis is an infection involving the colon, generally the extreme end portion known as the distal and sigmoid colon. The normal course of the disease involves low-grade fevers, loose bloody stools and abdominal pain followed shortly by relatively easy diagnosis, treatment with oral antibiotics and swift recovery. A previously healthy individual would certainly be laid up for a week or so, and a professional fighter might be unfit for high-level training for as long as a month. But Lesnar’s diverticulitis wasn’t diagnosed early, and he was absolutely right when he stated on "SportsCenter" that he very nearly died.
The complication Lesnar suffered was a bowel perforation, a pus-forming infection eroding through the wall of the colon. If the perforation is contained, medical treatment and percutaneous drainage generally result in full recovery. If the perforation is not contained, if large volumes of pus and stool spill into the abdominal cavity, then emergency surgery is the only hope for survival and death is an expected outcome. At best a patient could expect to lose all or part of his colon and be incapacitated for years. It appears, from what Lesnar explained on "SportsCenter," that he was diagnosed on the razor-thin border between these two extremes.
If he had gone to surgery, all or part of the colon would have been removed. The healthy bowel would be diverted to a hole in the abdominal wall -- a "fistula" -- so stool could have been drained into a colostomy bag. After a period of recovery, taking many months, it’s possible the colostomy could be reversed -- allowing normal function of the bowel -- but the recovery from this second surgery would again require several months. If reversing the colostomy were not feasible, Lesnar would have been bound to wear a colostomy bag for life; training for MMA, much less competing, would have been impossible.
The "lottery ticket" assessment of his recovery appears to be a function of Lesnar's abnormal physical conditioning. Athletes, generally speaking, are able to recover quicker than their sedentary counterparts in society. Wrestlers often have a particular set of antibodies developed after fighting off infectious diseases of various sorts from years of training and competing on the mats. Obviously whatever changes to the nutrients he was ingesting played a role as well.
Pitt also naturally raises a very relevant question: how is Lesnar's mental state and attitude after a frightening and arduous ordeal where everything he knew to be his life was thrown into peril? As Lesnar stated himself on ESPN, this is the first time in his adult life where he felt he didn't have control over the events and trajectory of his life.
Pitt argues he saw a more humbled Lesnar on SportsCenter, but I'm not so sure. While I do believe this entire process has rocked him to his core, overcoming adversity and getting on the path to normalcy can be the most important boost in creating a rejuvenated spirit. In the mind of an athlete, perseverance is a signal that nothing can stop them.