Doc Rivers Dumbfounded Disbelief Dafuq wtf face reaction gif
Adam Rifkin stashed this in Reaction GIFs!
stuff_of_epics says it's the quintessential disbelief gif.
Khao8 adds, "It's like he's trying so hard to make sense of it in his head, that his brain can't handle anything else other than breathing and blinking. Everything else is incapacitated because his brain can't process it."