When scouting new central defenders, managers take into account far more than the average fan reading tabloid links does.
Ezequiel Garay looks great on YouTube and Eliaquim Mangala is monstrous in the challenge, but that doesn't make them automatic fits for your team's defensive system based on talent alone.
Why? Because every coach strives for a relatively simple, yet difficult, balance to attain in central defence: a right-foot, left-foot combination to essentially split the pitch in two.
We've seen countless pairings at centre-half over the years that stand out as classics, true greats, and most are the product of a well-balanced approach divisible by the feet.
Milan's European Cup win in 1990 featured Franco Baresi (left) and Alessandro Costacurta (right) in defence; Marseille's '93 triumph saw Marcel Desailly (left) and Basile Boli (right) take centre stage; Ajax's 1996 team featured Frank de Boer (left) and Danny Blind (right) at the back.
History is littered with great duos, and the majority of them attain this simple balance that is so often overlooked. Why is it this can have such a dramatic effect on a team?
Basic Defensive Instincts
As a centre-back in a two-man partnership, much of what you do will be instinctive.
You move with the flow of the game, drop off when required and step in when you feel it's right to do so: The decisions take just microseconds in a player's head.
A left-sided centre-back (LCB) will regularly be dropping off to his left shoulder, and should he chase a wide man down the line will be looking to block/tackle with his outside foot.
Inverted wingers, who cut inside with great quickness and play across a slower defender's rotating body to find space, can sometimes catch a centre-back on their weaker side, but in that instance the go-to response is to force the player off balance and drive him into a midfield colleague.
Inverted centre-backs has not, and will not, catch on like it has with attackers.
Because of the instinctive nature of the vast majority of the decisions you will make, you naturally need to feel comfortable playing the ball first-time and clearing/playing out instantaneously. If you're leading with your weaker side, there's a much higher chance of something going awry.
John Terry, playing on the wrong side of defence for England at the 2010 FIFA World Cup, made several uncertain moves and got himself into trouble plenty of times. It's amazing how moving 15 yards left or right can throw a top professional off his game.
He's adept at playing either side, but the (short-term) culture shock of switching—due to injuries to the revolving partner of Ledley King, Jamie Carragher and Matthew Upson—scrambled his bearings.
The Passing Game
Half a decade ago, centre-backs who could pass out from the back became a real luxury once again. Now, more and more astute midfielders with good size (6' or taller) are being transformed into CBs every day.
It's gone from "lump it," to "It'd be nice if you can pass it" to "You need to be able to keep it on the deck." Borussia Dortmund under Jurgen Klopp are just one of many 2013-14 examples of how a ball-playing defender can make the difference in a central position.
Mats Hummels is the master of the long pass out of defence, and if you can rely on a centre-back to create via a 30- to 40-yard ball into a playmaker—therefore skipping between the lines and avoiding high pressing—you can negate pressure on your traditional midfield outlets.
When teams began keying on Ilkay Gundogan last season, the onus often fell on Hummels to find Marco Reus and Co. with longer passes. It worked, and his prowess in the passing game meant BVB were able to overcome stubborn sides such as Malaga in their fairytale run last year.
Another example presented itself on Tuesday, as Vincent Kompany fired an inch-perfect pass to David Silva between the lines. It beat Barcelona's four-man press with ease and put Silva one vs. one against Sergio Busquets in space, and the result was a chance for Alvaro Negredo.
Such wisdom on the ball has prompted some, including B/R's Jonathan Wilson, to suggest the time is ripe for the return of the libero.
This can't happen if you're on the wrong side of the central defensive pairing, as the angle is against you and you're forced to pass out, in line, with your weaker foot. Would Hummels and Kompany make these passes with their weaker feet? Not a chance.
It's not all tragic; not every centre-back placed on the wrong side fails immediately as if accidentally given pads, a helmet and slotted in at middle linebacker for the New England Patriots.
Laurent Koscielny is a right-footed CB playing on the left to accommodate Per Mertesacker while left-footed Thomas Vermaelen sits on the bench. It's not the norm, but Arsene Wenger has seen Koscielny is happy to play on the left, and in some ways he's adjusted his game to make it work.
But more often than not a defender can look incredibly uncomfortable, and that's why managers stock-pile defenders with different preferred feet. A prominent example of the ultimate balancing act is Paul Lambert collecting Ron Vlaar, Jores Okore (both right), Ciaran Clark and Nathan Baker (both left) for his central defensive corps.
The rise of the passing game from defence and the sort of impact players like Jan Vertonghen have on a game has now become so great, managers can rarely afford to play a wrong-sided centre-half.