Where Can Arsenal Improve Defensively in 2015?

If there's one reason why Arsenal are in fifth place right now, it is their defense.

The Gunners dropped far too many points at the beginning of the season, and they continue to do so on occasion, because of a lackluster and porous defense. A team that had one of the best defensive records in the Premier League last season has become just about toothless.

How did Arsenal's back line get so decrepit so quickly?

The answers will tell us who made what mistakes, and how the club can go about rectifying them.

As any goalkeeper will tell you, a cohesive back line is the unacknowledged key to maintaining a good defensive record. Playing with the same unit every week breeds confidence in one's knowledge of one's teammates and the coordination necessary to play the offside trap or move up while someone stays back.

A defense can only flourish when everyone knows his role and sticks to it with the knowledge that his teammates will pick up their slack. But Arsenal have lacked any measure of that this season, contributing to a back line that is visibly out of sorts.

What do you get when you combine a good left-back with a strained and slow central defender, a small left-back playing center-back and a 19-year-old one-year pro at right-back?

Exactly what you'd expect: a bunch of players ill-suited to their roles that crumble at the first strain.

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That is just what Arsenal have had for much of the campaign in Kieran Gibbs, Per Mertesacker, Nacho Monreal and Calum Chambers. All are not out of place at this level, but they were played too much and often in the wrong places.

It's looking up a bit now, largely due to the return of Laurent Koscielny. Not only is he an excellent center-back who can defend proactively and compensate for Mertesacker's physical limitations, he allows the rest of the defense to play in their normal positions—or even get a rest.

Arsenal have virtually never fielded their first-choice defense this season, and will not do so for some time, with the news that Mathieu Debuchy will be out for the foreseeable future after dislocating his shoulder against Stoke.

It will be up to the youngest players in the team to compensate.

Specifically, Hector Bellerin and Calum Chambers will compete for the backup right-back job—a contest that will no doubt do wonders for their development and spur each to work just a bit harder in training.

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But there will certainly be growing pains with both players, and the rest of the defense needs to slightly adjust to compensate.

Arsenal also have to add personnel. One injury to Mertesacker or Koscielny, and the squad is down to bare bones. As everyone knows, this is ludicrous and unacceptable. Yet there has been no sign that Wenger is close to bringing in reinforcements this month.

There does seem to be more buzz about the club bringing in a defensive midfielder, though. As long as the man the Gunners bring in is more focused on sitting back and protecting the defense than bombing forward like Alex Song, fans should be satisfied.

Mikel Arteta has barely played this season due to chronic calf problems. He has not been entirely convincing when he has played either, and is obviously not getting any younger.

Mathieu Flamini, meanwhile, has showed that he is far too reckless and technically unskilled to start on a regular basis. Most importantly, he frequently finds himself out of position and unable to recover in time to prevent waves of opponents from descending upon Arsenal's back line.

Francis Coquelin has been excellent in recent games, but has the Frenchman improved so drastically in such a short period of time to be a medium- to long-term solution? It would represent a rather incredible turnaround.

All Arsenal really need to do to lock down their defense again is reinforce two crucial positions: center-back and defensive midfielder.

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