I genuinely don't think it matters which and how many CB's Wenger bought or should have bought, no matter who they are, they need the correct structure, coaching, and balance across the team to perform at their best and for Arsenal to get back to keeping clean sheets again. I could be wrong but I don't envision Wenger analysing and disecting the opponents attacking strengths and weaknesses on the floppy disc all week prior to a game and instructing his team accordingly, nor do I envision him out on the training pitch coaching defensive work on positioning, attitude towards defending, tackling and aerial techniques etc. We can argue about who they should or shouldn't buy but the defenders they've got now aren't bad players, at least not as bad as they are performing currently. Vermealen is class, Koscielny started last season with great promise and got called up to the French side, Djourou looked to be fulfilling his potential prior to CC disaster, and Mertesacker has been capped 70 odd times for Germany and been involved in two successive runs to the WC semis, the calibre of the defenders in question is of a good enough standard to be doing a lot, lot better than they have been and avoiding the basic errors both in marking and concentration that just keeps happening far too often for it to be coincidence.
First things first, they need a defensive coach who knows how to organise a defence and who can get them all singing from the same hymn sheet(they aren't playing as a unit, too many cases of one or two playing offside, the others not, lack of communication etc), it's clear as day that not enough is being done on the training ground to eliminate these basic errors we've been seeing for years, in fact it's getting worse from week to week.
Secondly, they need an enforcer in CM who'll just sit and sweep up in front of that back four, as badly organised as they are, no defence could withstand such a lack of protection they have to work with, Wenger sets the team up far too open and has no player in front of his defence with a defensive instinct in his body and a discipline to protect his own goal.
In short, if Arsenal want to get into CL next season and want to start competing again, Wenger needs to get a proper defensive coach on the books, someone who'll restore some confidence and organisation to the current defenders they have, and he also needs to be more tactically balanced with his team selections(the gameplan at OT was a joke considering the personnel he had available but again he failed to address the strengths of the opponents), then and only then will signing this centre-half and that centre-half make a difference.