Wojciech Szczesny Errors Against Southampton Open Door for David Ospina

For much of this season, Arsenal fans have been told that defensive stability would return once they were able to field their first-choice back five. In this defeat to Southampton, that assertion was exposed as a fallacy.

Even with Laurent Koscielny and Mathieu Debuchy restored to the starting XI, Arsenal's defensive unit spent much of this match in chaos. Their gritty win at West Ham United was surprising at the time—now, it looks like a full-blown Christmas miracle.

The guiltiest party was Wojciech Szczesny, who made obvious errors on both Southampton goals. For their first, he charged irresponsibly from his goal, allowing Sadio Mane to clip into an empty net. Szczesny's disastrous dash brought back painful memories of mistakes from Manuel Almunia and Lukasz Fabianski. 

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On the second goal, he inexplicably poked a clearance straight to Dusan Tadic, who punished him with a neat finish inside the near post. For Szczesny, this was undoubtedly a day to forget—if he can.

After this disastrous performance, Szczesny may find himself out of favour at the Emirates Stadium. Arsene Wenger has shown a willingness to leave him out the side when he feels the Pole's focus has dropped, and Szczesny could be set for a spell on the bench to atone for his errors.

Wenger has already confirmed that Colombian international David Ospina will start the FA Cup third-round match with Hull City. Ospina has spent much of the season out injured, but he'll surely sense Szczesny's vulnerability and be keen to seize his opportunity to shine. If Ospina keeps a clean sheet against Steve Bruce's side, he may retain his place for the visit of Stoke seven days later.

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However, it's important to note that although Szczesny will take the brunt of criticism, his back four were not blameless on either goal. Koscielny allowed Mane to escape him far too easily for the opener, and Debuchy could arguably have cleared the ball instead of poking it back to Szczesny to prompt his feeble clearance.

Arsenal's problems weren't only in defence. The back four were exposed by a lack of protection in front of them. As last season, Arsenal's midfield has been ravaged by injuries. At St. Mary's, Arsenal were forced to line up with Calum Chambers and Francis Coquelin in holding midfield. Coquelin battled well but does not convince as a long-term option in that role. For Chambers, it was the first time he had ever been picked to play that role in a senior game.

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There were issues up front, too. With Danny Welbeck ruled out with a thigh injury, the folly of Olivier Giroud's needless red card against Queens Park Rangers was exposed. Without the Frenchman, Arsenal lacked a presence up front. Alexis Sanchez worked tirelessly but often looked isolated. Arsenal have been accused of being a one-man team—at St. Mary's, in attack at least, they really looked it.

Despite their elevated league position, Southampton have tended to struggle against elite opposition this season. This result is yet another indication that Arsenal don't quite belong in that company.

James McNicholas is Bleacher Report's lead Arsenal correspondent and is following the club from a London base throughout the 2014-15 season. Follow him on Twitter here.