#3 Laurent Koscielny
Frenchman Laurent Koscielny had become one of the best defenders in the Premier League over the course of the past few years and naturally, he is the player around whom Arsenal's defensive game revolves. The 5-1 collapse against Bayern Munich last season after he went off at 1-1 is a case in point.
So, even if his game drops by a bit, the effects on Arsenal's overall play is profound and this season has been a particularly trying campaign for the Frenchman, who has simply not been at his best. His Achilles injury has also flared up and he needs treatment for it every day, which has made him a liability of sorts when the club needs to play more than 1 game a week.
The mistakes this season have piled up at an alarming pace and what should be even more worrisome for Arsenal is that his famed pace also seems to be going south. Additionally, the way he got outmuscled by Paul Pogba in the game against Manchester United which directly resulted in United's 3rd goal in the game was a clear indication of the fact that Koscielny is no longer the defender that he used to be.
In the most recent game against Liverpool, his mistakes cost Arsenal a goal as well. That is not the sort of mistakes that can be tolerated by the defensive leader of a team like Arsenal and perhaps it is time that the club started looking for a new central defender or two.