#6 Manchester United
The fact that the sixth team part of the "Big Six" isn't even in the Top 6 gives you an idea of their weaknesses. United have had a turbulent campaign that has had some highs, but the lows have been significantly longer and have been more evident.
Pointing out weaknesses in a side that's going through a rough patch is easy and yes, United are looking defensively vulnerable, barring Pogba, they have no creative outlet in midfield and their strikers haven't been clinical. These are all evident and easy to point out but, the one weakness that is really haunting Manchester United is the fact that they have no identity.
Jose Mourinho is tinkering with his team too much and it seems like the players haven't been given key instructions or even have assigned roles. The forwards on some days drop deep and try to hit on the counter and on some days play a very direct game. Mourinho seems to have confused his players and really needs to bring some clarity to the team if they're to salvage anything from this season.