From memory when they sold Nasri they had an injury crisis and a vital CL qualifier which is why he played.
A personal opinion is that if Wenger thinks Sanchez is off he won't play him, he will want to show confidence in the players he has and get them more used to playing together. If Sanchez starts I suspect this transfer is doubtful save a huge bid from City.
I tend to the view that Arsenal are basically going through the motions to placate their fans. I wouldnt be surprised if Wenger played Sanchez half hoping he looks disinterested. Wenger than give a "what can you do?" shrug and very, very, very reluctantly agree to sell him.