That illness really hurt Lesnar, he was never like that before. He used to take punches fine before that illness got to him, he took a massive knee in the Mir fight and was fine.
Brock may have been fine eventually, but it took him 45 seconds of clinging to Mir's legs (reminded me of a kid holding on to their mother) before he moved - of course when he moved, it was to finish the fight. However, I believe his issues with being hit precede his colon issues.
Unless Brock is prepared to spend a 1,000+ rounds sparing with partners who are allowed to hit him, he shouldn't come back. He no longer has the menace and fear factor he used to - everyone knows he turtles when hit and nobody is scared to face him...
That being said, if there is a chance that will fight again, Dana will have him back and headlining a fight card in a heartbeat – Brock might be scared to be hit, but he can be guaranteed to brings the most important (to the UFC) thing of all – PPV buys!