#1 Defensive mistakes
Probably the most pressing concern around Old Trafford right now is the number of goals the team has conceded in just over two months in the current campaign. Jose Mourinho's teams are built on solid defence, and notably, the manager did instil the same mindset among the players when he signed Eric Bailly and Victor Lindelof respectively in two different seasons.
The previous two seasons United has been quite stable at the back. In their last campaign, they were only behind winners Manchester City regarding the goals conceded. But that defence has turned worse in a matter of few months.
The goals against count have already doubled what it was at the same stage previous season. Even the defensive midfielders like Matic is not doing enough to protect the back four, and there are a lot of spaces opening up between the defence and the midfield. Jose himself stated that he doesn't know his best centre-back pairing yet.
Mourinho wanted a central defender this summer, and his request has reportedly been turned down by the board citing the lack of big-name players that Jose was after and the failed transfers from the previous seasons. Against small teams, it's still possible to come back and score after going one or two goals down, but against top sides like Juventus, it's very tough to win after conceding.