Look i'm not gonna say the illness did not affect him, but come on now. Withered, faded, and ruined? That's really pushing it.
Brock never liked getting hit in the first place. That was obvious from the get go. Up against someone like Overeem who he can't simply out muscle and bring to the ground he was left with no recourse but to strike with a much better striker. How exactly would this fight have gone differently in 08/09? Brock certainly gained skill from 08/09 so do you really think that a small loss in muscle mass would really break Lesnar, or ruin him, as you put it?
The fact is Lesnar was never good, he just came to the UFC at a perfect time and got perfect opponents. Let's look at his record:
Herring: Average fighter, much smaller than Brock, was on the way out of the MMa game.
Couture: Completely undersized compared to Brock. Literally the size difference is the same as Machida fighting Bendo. There was like a 40 pound size difference. Oh, and let's not forget Couture was an old man.
Mir: Lesnars one shining victory. Although he still used size and strength, he did show some good ground technique. I sign that he is improving, at least a bit. Lesnar also lost to Mir the first time.
Carwin: Practically a striking clone of Brock. Carwin is a terrible fighter that relies on size and power forgoing technique. Brock barely won, too. Carwin had him near finished and few would have argued if the fight was stopped. Brock showed how average he really was in this fight.
Cain: Lesnar got exposed. A much smaller man with much, much, much more skill makes him look silly. Before his illness "ruined" him, as you put it.
Overeem: Lesnar is up against someone his own size for the first time. The only problem is Overeem is a much, much better striker, and since Lesnar can't simply use his size to drag Overeem down, we got the result we did: Lesnar was forced to strike and got exposed again.