The main reason that Arsenal were not at the races vs Chelsea

 By 1970s Gooner 

Arsenal lacked control for 76 or so minutes in the game against Chelsea. The main reason for this was the deployment of Jorginho at the base of midfield. 

This meant that Rice was moved into the Xhaka role on the left. Also Zinchenko's invertion into midfield was not that apparent as he had to content with the threatening runs of Sterling on that side.

Chelsea on the other hand played with a false 9 in Palmer which meant that in their half block they had one if not more players in the middle than Arsenal. 

More importantly Jorginho was not at the races himself as he was not able to provide a decent defensive barrier nor was he able to pass the ball forward that often preferring to pass sideways or backwards.

All this  was corrected when Jorginho was substituted and Rice was moved in the anchor position.

It appears that Arteta had overestimated Jorginho's ability to control the game by repeating his fine performance vs City. He didn't of course. 

Now why Arteta didn't start Partey there is beyond me and many many Arsenal supporters.

If Arteta felt that Partey was too tired from his travels to the USA then he could have started Rice as the number 6 and brought Zinchenko as the left sided midfielder with Tomiyasu at left back.

Now that's an idea...