#2 Lack of protection from midfield

The issue starts with the two wide midfielders. There is no doubt that Ngolo Kante is one of the best defensive midfielders in the world, as there is no better ball-winner in the world.
However, if there is one weakness in his game, it his tendency to fail to track a midfield runner - as it was visible in Manchester United's second goal where he failed to tackle Pogba and allowed him a free run in the box. Although it should be remembered he is learning a new system and position, and one can hope for him to improve in that department.
Similarly, Matteo Kovacic is also someone who struggles with this aspect of the game. Malmo's goal in the first leg of Chelsea's Europa League match and Herrera's goal for Manchester United in the FA Cup exhibit his weakness at keeping runners in check. Even Ross Barkley is not doing a better job when he plays.
Ultimately, responsibility must fall on Jorginho. The Italian is an extremely intelligent player and only Leicester's Ndidi has won possession more than him in the middle third in the Premier League.
However, the Italian is someone who is not the quickest or strongest. This makes it very easy for opposition players to shrug the ball off him or run past him. Its evident that the midfield is not offering enough protection to the back four.