Yes of course course we should expect our central defender to not be so easily beaten, but a keeper cannot assume everything will be ok in a 1v1 situation. Onana has to know that his 36 year defender is vulnerable to being beaten by a 26 year old attacking player. At the moment Onana and the rest of the world saw that Evans was struggling, Onana had to come out which in this case meant only two steps, not ten yards. Instead, Onana remained rooted to his line and once he froze on his line he had no chance of stopping the shot.
What makes Allison and Ederson, the standard by which Onana must be judged, elite keepers is their ability to see danger and proactively close the angle down. Both Alisson and Ederdon are elite shot stoppers and Onana, unfortunately for United, is not.