Brock is never going to be champion again. I've been saying this for a long time now but his wrestling is vastly over rated and his striking is really quite poor.
The days of one dimensional champs are long over, the sport has evolved. Brock isnt going to suddenly become some pro boxer/kickboxer in the span of a year or two and i dont think hes ever going to react well to getting punched in the face. Being honest here the only other fighter i have ever seen react so badly to getting hit hard is Bob Sapp. And look how long Sapp has been fighting for, he still hasnt learned how to take a punch.
Some people are born to fight, some arn't. Lesnar is a wrestler, not a fighter.