#1 The lack of defensive presence in Chelsea's midfield
Chelsea started with Jorginho and Mateo Kovacic in a midfield two and it looked like the Blues lacked the defensive presence of somebody like an N'Golo Kante in the middle of the park.
Whenever United counter attacked, the lack of pace of both Jorginho and Kovacic meant that they weren't going to catch up to the United forwards and that helped the home side open up Chelsea's defense at will. The thing is, Kante should have started this game even if he wasn't a hundred percent fit as the Frenchman offers so much off the ball.
Also, Jorginho has to improve defensively if he is to play in that number six role as Kovacic is clearly seen as the more attacking option by Lampard. If this match was anything to go by, Chelsea have to work a lot on the defensive side of their game, including in the center of the park.