In the end, they bumped the good doctor not because they overbooked, or they had to seat a United crew.
They booted him out because he disobeyed legal instructions from the flight crew and became disruptive.
IMO the good doctor was no better than an unruly drunk that creates a scene and is also removed.
The fact that he was beat up was on him.
UA broke the law by evicting him.
And it is not clear to me that an air-crew even gave the order. (Gate folks are not air-crew)