[
You're right Manchester United will have to sign more extra defenders before the transfer windows closes, they will need additional defenders left to substitute any fellow injured defenders. I also agree that Reuben Amorim still has to orientate his players to motivate them and to also increased their team spirits. Yeah Arsenal were able to defeat the Manchester United with just a goal by penetrating United defense doesn't also implies that Manchester United defense is so weak. I think Manchester United defense is still strong but definitely they should try and sign additional defenders.
No, Man Utd don't really need an additional defender, as they already have enough players in that area. The only player they need to add to their squad is a goalkeeper and that what they should be thinking about now and starting to accelerate their goalkeeping signings before the transfer window closes.
However, the good news is that they've shown significant progress in attack, which has been very useful this season. Amorim just needs to motivate and instill in the players the mentality to bounce back when they fall behind, which they didn't show against Arsenal last week.