The problem with Manchester united defenders is that they can not build an attack from behind. They will slow down the ball, pass back to goal and continue back pass until the opponent recovers the ball from them. Only Dalot is exempted from this terrible style of play. However, Manchester united problem is not only limited to this two areas you mentioned. There is problem with their midfield too because in most games, both Casemiro and Bruno completely disappeared from the game and making it difficult for attackers to play outside their own half since midfielders can not defend or help to cover the defenders, this result to wingers falling back to support. The couch too is part of the problem because he has proven how clueless he is since he has no tactics to rotate the squad and give players time to rest so they can be able to give their best when they are well fit.