Manchester United still have a big problem to tackle especially, corner kicks. Last season, United conceded several goals from corner kicks and this season, it looks like the same thing might happen. Amorim should work on the players, make them take full responsibility of defending the post and macking their opponents whenever, there is a corner kick.
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, just the way Manchester United struggled to defeat Burnley at Old Trafford before the international break.
The problem is not where they concede goals from but the way they concede goals and the teams they concede against, the defense of the team is the biggest challenge right now the team is doing well in the attack if they keep conceding more than the goals they score it's going to be difficult for them to win matches. Against Burnley in Old Trafford it was a poor performance in the defense Amorim can do better if he tackles the defense errors and motivate the attacking force to take their chances.