Although the statistics show that Juan Mata seems the best overall, just the goals and assists don't tell the full story. The data is skewed slightly by two factors. The first is that Juan Mata was Chelsea's primary free-kick and corner-kick taker last season. The others were involved in set pieces, but not to the extent of Mata.
The second, and more important, is the time spent on the field. Obviously playing more minutes increases the chances of getting more goals and assists. Next I will be comparing goals and assists against appearances. Unfortunately I could not find the exact amount of minutes each player played, so I will count starting appearances as 1 game and substitute appearances as 1/3 of a game.
I won't be looking at it in this article, but I think Wenger could have a difficult time choosing his best midfield and wings if everybody is fit.