If you've watched enough Barcelona this season you're probably very aware of their problems on the back line. Barca have not been known for their defensive prowess over the past few seasons but it hasn't stopped them from soaring to great heights both in the league and in Europe.
Their "devil may care" defensive attitude and seemingly suicidal use of their fullbacks as highly offensive wing backs leaves their central defense duo nearly alone at the back to deal with the opposition's attacking forays. It started to be less effective last season and the wheels have flown off this season.
The problem isn't really shielded in mystery of course as regular injuries to, and the general degrading of Carles Puyol's ability has thrown a sabot in the defensive machine.
In the past Puyol has been the unshakable foundation of Barcelona's defense. Imagine him as an incredibly strong pillar holding up a house, it doesn't look stable, it probably shouldn't work, but because of Puyol being able to do exactly the job he was needed to do, it did work.
Without Puyol in the lineup, or in most cases even with him in it, the pillar has eroded and cracked, becoming weakened and unable to properly support the house. Javier Mascherano was alright but he really isn't the type of defender to play the Puyol role. Alex Song certainly isn't the type of defender to fill the role and neither is the recently returned Eric Abidal (though he could help temporarily). Marc Barta might be the guy one day but he's certainly not there yet.
There's nobody on the roster who can replace Puyol and when the Barca captain is available, he's not what he used to be.
Barcelona are left with two options. Either they sign a central defender in the summer that they believe can be a defensive anchor and allow Piqué and the fullbacks to do their thing, or they change the whole system. The second option doesn't seem likely because Barcelona's system is now engrained in the club and I can't see them making a titanic shift without bringing in an entire new coaching staff.
So that leaves signing someone.
Plenty of names have been thrown out for Barcelona to consider including Manchester City's Vincent Kompany or Borussia Dortmund's Mats Hummels. There are others of course, those just happen to be two of the biggest names. It really doesn't matter who Barcelona sign as long as sign the right guy. They need a defender who can do what Puyol did so well for the last few years and if they don't sign the right player, it's hard to see their defensive woes somehow magically getting better.
They also need to not miss out on getting a player they so desperately need. Barcelona either failed to go after or just whiffed on Thiago Silva and Javi Martinez last summer and they cannot afford to see another transfer window go by with no defender brought in. A stubborn belief that Puyol can find his past form is going to cripple Tito Vilanova going forward.
Don't forget about Victor Valdés and the his apparent decision to leave the club. Finding a proper central defender will become even more important to help deal with the integration of the inevitable new goalkeeper signing that will come as well.
If Barca sign another Alex Song-type, their issues will continue to get worse. If they sign a Puyol-esque (in his prime) pillar of defensive awesomeness, things should settle down at the back for the Blaugrana.