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.
What you say is true, because corners are unpredictable, so what Ruben Amorim is doing to improve Manchester United's defense is not wrong at all. Manchester United's midfield and attack aren't too bad either if they can all support each other to improve in the upcoming matches this season. Oh yes, will goalkeeper Onana be loaned to Turkey by Manchester United? I've read a bit about that recently, and if it's true, I think Manchester United will definitely improve this season, because other goalkeepers at Manchester United probably won't make the same blunders as Onana.