I am not sure if the chin is the only problem. Several fighters said the same about the Reem, and BigFoot Silva put it bluntly into words: "Overeem is a lion when is punching, but a little kitty when is getting hit".
I don't blame him entirely, getting hit by heavyweights must be a unpleasant experience (i was hit hard doing boxing with featherweights and jr. welters, and it is not nice), but Reem sems to shut himself down mentally after a couple of punches. My father, a former pro featherweight boxer, used to say that if you want to have a measure of a dude in the ring you need to see how he reacts when he is under pressure and getting tagged. Cerrone is a good example of that, you can defeat him but you can't intimidate him with a couple of punches. When Reem starts feeling the knuckle sandwiches coming in, he just fold down and that's it.
Maybe it is because he knows that his chin will be unable to cope, but there is something else in his attitude, I think.