I thought it would be interesting to break down the defensive stats and try to see where the problem is, if indeed there is a problem.
2012/13 - We conceded 1.103 goals per game (Let in 32 in 29 games)
19 Games - Szczęsny
9 Games - Mannone
1 Game - Fabiański
2011/12 - We conceded 1.289 goals per game (Let in 49 in 38 games)
38 Games - Szczęsny
2010/11 - We conceded 1.132 goals per game (Let in 43 in 38 games)
15 Games - Szczęsny
14 Games - Fabiański
8 Games - Almunia
1 Game - Lehmann
2009/10 - We conceded 1.078 goals per game (Let in 41 in 38 games)
29 Games - Almunia
5 Games - Mannone
4 Games - Fabiański
2008/09 - We conceded 0.973 goals per game (Let in 37 in 38 games)
32 Games - Almunia
6 Games - Fabiański
1 Game - Mannone
*This adds up to 39 as Fabiański had 1 game as a sub
2007/08 - We conceded 0.815 goals per game (Let in 31 in 38 games)
29 Games - Almunia
7 Games - Lehmann
3 Games - Fabiański
*This adds up to 39 as Lehmann had 1 game as a sub
2006/07 - We conceded 0.921 goals per game (Let in 35 in 38 games)
36 Games - Almunia
1 Game - Lehmann
1 Game - Poom
2005/06 - We conceded 0.815 goals per game (Let in 31 in 38 games)
38 Games - Lehmann
2004/05 - We conceded 0.947 goals per game (Let in 36 in 38 games)
28 Games - Lehmann
10 Games - Almunia
2004/05 - We conceded 0.684 goals per game (Let in 26 in 38 games)
38 Games - Lehmann
As you can see from the goals conceded per game that its slowly rising season by season
I have also looked at all the central defenders Wenger has bought in the last decade or so and give my own view on whether they were any good or not
Per Mertesacker - SV Werder Bremen (Germany) - £8,000,000 ... Good
Sebastien Squillaci - Sevilla (Spain) - £3,300,000 ... Very Very Poor
Laurent Koscielny - Lorient (France) - £8,500,000 ... Good
Sol Campbell - Free Agent - Free ... Very Good
Thomas Vermaelen - Ajax (Holland) - £10,100,000 ... Was Superb Now Good
Havard Nordtveit - FK Haugesund (Norway) - £2,000,000 ... Never Used
William Gallas - Chelsea - £5,000,000 (Swap Deal With Ashley Cole + Cash) ... Very Good
Johan Djourou - Etoile Carouge Jugend (Switzerland) - Free ... Average
Philippe Senderos - Servette (Switzerland) - £500,000 ... Average
Pascal Cygan - Lille (France) - £2,100,000 ... Poor
Kolo Touré - ASEC Mimosas (Ivory Coast) - £150,000 ... Legendary
Efstathios Tavlaridis - GS Iraklis Saloniki (Greece) - £600,000 ... Never Used
Sol Campbell - spurzzz - Free ... Legendary
Igors Stepanovs - Skonto Riga (Latvia) - £1,000,000 ... Very Poor
So in 13 years he has bought 14 central defenders of which 2 were top quality players both of which were ready made and then Kolo turned out to be top quality, 2 more are still debatable (Verm & Kos) and the rest are all below the standard we expect at Arsenal
Ray Parlour was on TalkShite and said he knows for a fact that Wenger does fuck all defensive training like in the old days and that when players would be good when they join then learn the Arsenal way that does not happen any more
Then Steve Lomas was saying the same thing a few days later saying Wenger has no interest in the defensive side of coaching