He played well, its just Arsenal are a mess of a club right now. Until they get a back line that can play a passing system, you will continue to see this sort of result for Arsenal unfortunately. Why Wenger thought Mertersacker could ever succeed in his type of system is beyond me and his lack of pace, slow decision making and reading of the game cost Arsenal on the break.
As far as Alexis vs Suarez, its a terrible debate to begin with because you are comparing two different type of attacking players. Alexis is much more of a winger and Suarez much more a striker and whether or not one is a better fit than the other, in my opinion, has nothing to do with their characteristics or personalities as a player of one being better than the other. Barca needed someone a bit more clinical than Alexis but while Suarez is more clinical, Alexis is quicker and faster.
Both are great players and both have and will do great things for this club.
Arsenal just need a proper holder and some ball playing CBs and then they will be okay but this has been a glaring deficiency in this squad almost since Gallas left a few years ago and Wenger
still won't make a move in that direction in the window