#1 Torreira's presence
Despite playing just the second half, Torreira has been absolutely influential. The Urugyuan has made the difference and though he has not assisted or scored any of the goals, the massive amount of energy and enthusiasm he brings onto the pitch has been decisive for the Gunners.
Torreira's first pass of the game was a clear-cut through ball for Lacazette to score. The first airball by Torreira for the Gunners was against City, for Aubameyang, which he ended up scoring, but was unfortunately flagged offside.
Mkhitaryan is having a pivotal role in the squad, which is to hold the attacking line-up together, as Ozil has been a bit inconsistent for a few seasons now.
Torreira, being a CDM, plays killer passes to his forwards which leads to goal scoring chances. This takes a bit pressure off the likes of Mkhitaryan and Ozil.
It is the boss who decides who plays, and he certainly has seen Torreira's performance in all the three games, coming off from the bench. Probably, Unai Emery is planning something big for the fans.