Initially, I was surprised but as I thought more about it....
I don't think it will be too bad. If Doc Rivers, who was an awful defensive coach before Thibs came along, be the head of a competent defensive team - than why can't the converse be true for Mike Brown? Get him a top quality offensive assistant and he'll be fine.
Secondly, for all of Mike Brown's lack of offensive IQ, FWIW, he never had any personnel to do anything with. The Cavs second best offensive player was Mo Williams.
Mo Williams.
What kind of offense are you going to design around that other than run the whole thing through LeBron?
This won't be as bad as people think.