I would be very wary of buying into this hype. I am about 90% certain this is just Dana trying desperately to build up some giant comeback for Brock.
The truth is Diverticulitis is very treatable. Frankly, the idea that someone in good physical condition, with essentially limitless funds for health care, (ie, Lesnar) could have their career ended is extremely unlikely.
I'm not saying Diverticulitis is a walk in the park, it isn't, it is painful and nasty, but the only real factor for Lesnar should be time. Given enough time to train and recover, he should be 100% again with no real difficulty. Dana just wants to make it sound like he recovered from terminal brain cancer, aids and brain sucking aliens all at the same time to hype his return. Odds are overwhelming that he will be fine, it will just take awhile.