Any club can concede a goal from a corner kick, so that's not a problem that Ruben Amorim should bother himself to look into. It is okay if Ruben Amorim works on Manchester United's defense so that they will stop conceding unnecessary goals that will put Manchester United in difficult situation of winning their matches,
Even with the greatest defenders in the world joined together there is always a chance of scoring through corner kick because there are some corner that are beyond players or defenders interception do to were it is being placed, I have seen some that even the goalkeeper could not reach it, so actually except it is obvious that defenders was right in the place they should have defended well before they could take a blame for such scoring, free kick is also another one that might not also be caused by a player if scored because just like the above explained is also a direct kick to the post so anything is possible, depending on the choice of the player.