Amorim has been able to tackle the problem of the attack though they still haven't done any significant thing in the game because the team is performing well up front but no goals to show for it but the defense line needs a serious work because the team can't be conceding cheap goals when they are still still finding it difficult to score even against weak opponents.
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.