We dominated the first 35 mins and should've scored more than 1 goal. After that, there was an apparent drop in the energy levels and the players looked tired. This could be due to back to back Christmas fixtures and some players including Laca, Nelson, and Ozil not getting much game time this season. From that point forward we barely created anything and the team started to defend deeper as a result so I don't think it was a conscious decision by Arteta to do this. He spoke about it in his post-match interview and this was something he wasn't happy about.
Regardless, even though Chelsea had a lot of the ball the defending was compact. Chelsea never threatened and struggled to do anything with ball. Unfortunately, individual mistakes by Leno for the first and Mustafi for the second cost.
I think what people haven't noticed is that we're missing 3 of our center backs, our 2 left backs and our right back. We were forced to play an 18 year old winger as a LB, our 5th choice CB and a CM as a RB. With Holding, Sokaritis, Martinelli and Bellerin all back in training, hopefully that should give Arteta something to work with. Main concern is the lack of goals upfront with Laca and Pepe out of form. We're relying too much on Auba and an 18 year old Martinelli.