Chelsea were the better team in the first half when we were playing 4 at the back and we were lucky to be at 1-1. We switched to a back 3 with Elneny dropping between Koscielny and Mustafi in the 2nd half, and were the better team. So a return to a 4-3-3 may have helped in the Palace game, but clearly not the Chelsea game. The return of experienced players has definitely helped though, Ozil has come back for these 2 games as well remember.
However the big thing is that it's been quite noticeable that the entire team have a bit more fight in them for the last 2 games, best evidenced by the team effort to recover the ball when we lose it. That's what has come back, and will hopefully carry on. We just didn't have that recently. Wenger also discussed this in the press conference, saying that our performances in January have suffered due to a lack of clarity over the playing squad, and that that appears to have improved since we sorted it out.
All the evidence suggests that there was a rift in our dressing room, and that Sanchez was the key player in it. Players not celebrating his goals with him before he left, Elneny's tweets, the total absence of any well-wishes from any of our squad when he left (contrasted with their widespread goodbye messages to Coquelin and Walcott), Wenger's comments, I feel it's all pretty obvious, and that surely has to affect performances in at least a small way.