You keep saying that despite it not being true. Dalot was covering Werner on Spurs corners. On the corners that the ball was dropping short he would then drop back onto the goal line to cover more space that the keeper couldn't reach. Doing that saved one goal, but then Spurs had another almost identical chance from the opposite side which unfortunately was just out of Dalot's reach. He was never 'on the back post'.
Dalot was not covering anyone. At the moment the corner kick was taken he was standing one yard outside the post marking no one, which was criminal to begin with, and once Richarlison headed the ball, Dalot six one yard inside the post marking no one, a second felony count.
The point here is that Onana was not guilty of a criminal offense on the Richarlison goal.
We could take a different tack to find Onana’s guilt, which is to argue that he should have come out for the cross but that would be asking too much of Onana. Onana now or De Gea before him, United keepers over the last decade have not come out on corner kicks. Criminal liability for the Richarlison goal falls mostly on Dalot for not protecting the back post, where he actually was not defending anyone, nor on Onana, who was positioned where he and De Gea before were instructed to be.
There was nothing “unfortunate” about the Richarlison goal other than Dalot defending no one or protecting the back post. Only in that sense was the goal…”unfortunate”.