So he contacted the doctor again and again to be assured there was minimal risk of further damage.Nope. Sanderson's comments show clear as day that it's up to the coaches if they take a player out of the game in this situation. Whether that is as a sub or not is irrelevant. Do you think no players were pulled from the game before the sub rule came into play? Of course not.
As Sanderson's comments make clear, there was debate among the coaches as to whether Dangerfield should play on or not. It was their call to make. He consulted the doctor a number of times, not for the doctor's decision, but for his opinion on whether this type of injury could sustain further damage. Even when he had the doctors assurance, he still wasn't sure about leaving him on and contacted the doctor again and again. This was his decision making process and the decision was his to make.
Leaving Dangerfield on was ultimately the coach's call. Sanderson had every power to take the player off to avoid what he himself called "any unnecessary risk" and no one would have batted an eyelid if he had done so; quite the opposite of this raucous "he can't override the doctor, you don't understand how modern football works Monkey King" chorus you've all cried out in this thread.
Sounds like the doctor's call to me.