#2 Lack of experience at the heart of defence

Irrespective of the defensive abilities and age factor concerning the now-departed David Luiz, it's important to note that his experience, leadership skills and ability to create a partnership with his fellow center back was dearly missed by Chelsea. Not to mention, Antonio Rudiger is still sidelined as well.
Andreas Christensen and Kurt Zouma were plotted in the middle, and to say the least, the pair probably experienced their worst outing in a blue shirt. Just when it looked like the away side were in control of the game, Zouma while playing out from the back, gave a pass to Martial, who fired tamely at Kepa. Matters worsened for the Frenchman when he aimlessly caught Marcus Rashford's trailing leg in the 18-yard box. The latter stepped up imperiously and gave his side the lead.
Christensen was tasked with Martial's run during the build-up to the second goal, but he remained in a zonal position, leaving Cesar Azpilicueta with herculean man-marking to do.
It'll take time for both these defenders who have astute abilities but Chelsea need Rudiger's experience and alertness to return.
#1 Inability to soak the counter-attack

Against a spirited Manchester United side, it was interesting to see how pragmatic and tactical Lampard's approach would be. However, inside the first five minutes, the likes of Pedro, Emerson, Mount and Abraham gave us a clear idea - they didn't arrive at Old Trafford to sit back.
The Blues looked far sharper than their counterparts, but what stung them deeper than anything else was how they were caught out during possession. Each goal constituted of a different type of schoolboy error, starting with Zouma, then Christensen and lastly, captain Azpilicueta.
Take nothing away from Manchester United, who were as brutal as it could get. But Chelsea had to, and will have to, defend in larger numbers in midfield against top quality oppositions.
Flair and flavor are two ingredients of the game that excite fans and everyone around, but tactical, defensive knowledge during possession is probably the most important deployment against Premier League sides.