I have been as forgiving as the officials (thinking they usually make the right call) this year, but, to me this was not a flag. The problem with vague rules like "defenseless receiver" is it means nothing. A guy can be defenseless if his back is to the defender, if he is running toward a defender with his head turned....I guess the defender is just supposed to let him catch the ball. Basically the rule is enforced if a hit is hard and looks like a guy might get hurt it is flagged, doesn't matter if it is legal or not. Asking refs to be able to distinguish between a clean hit and an unintentional helmet to helmet hit at games speeds, often from bad angles, is just silly. Come on NFL, do something smart for once a make plays like this open to quick review. NBA reviews take a few seconds generally.
This was a clean hit to me.