Manchester United needs defenders more than they do for midfielders, last season their defence was a piece of shit and to crown it all they don't even have a quality goalkeeper that is dependable but erratic in the goalpost instead. A midfielder shouldn't be Manchester United priority if they wish to sign a new player, adding to the the defence and goalie would bring a more wholesome strong depth instead.
A defender wouldn't hurt them, but in the last game they conceded one goal against Arsenal, which I think is still acceptable and not a sign of a broken defense. Their game in the offense wasn't good enough to overcome Arsenal and that on the other hand didn't have much to do with how Manchester United's defense did in that match.
Perhaps their midfield is a little bit too offensive at times as they don't have a midfielder who likes to overtake important tasks in the defense. But so far it has been one match, I will still give it a few weeks and see how good they really are.
It's just a one game in 38 games to play in the season, Manchester United first match was one which they played as the better side and with such game when you're the better team the weakness in your defense can't be exposed not till you meet a side that is better side over you. I think if it was Arsenal that were dominating that match against Manchester United, it would have been the dominant Arsenal team that would have won the game .