When Claudio Ranieri's Leicester City won the Premier League in 2015-16, their first-choice full-back pairing of Danny Simpson and Christian Fuchs cost a combined £2 million. As Pep Guardiola's Manchester City attempt to win the Premier League in 2017-18, their first-choice full-back pairing of Kyle Walker and Benjamin Mendy cost a combined £95m.
This is an extraordinary statistic that says so much about so much: the improbability of Leicester's triumph, the sheer amount of money Guardiola has been forced to spend to rejuvenate City and the extraordinary inflation in transfer fees over the past three years. More than anything, however, it demonstrates the huge difference in the transfer fees for old-school, defensive-minded full-backs and the more attack-minded, rampaging overlappers upon whom top clubs have come to depend.
Almost since the position was invented, full-back has been football's least fashionable position. Gianluca Vialli once said the right-back was generally the weakest player in a side; Jamie Carragher more recently opined that all full-backs were either failed wingers or failed centre-backs.
That largely remains true. It's extremely rare to encounter a team whose best footballer is their full-back: they are generally playing there because they're not capable enough to excel in another role, whether they adjusted in their formative years or during a midcareer switch. It's notable that many of the world's best full-backs in recent years -- Marcelo, David Alaba and Philipp Lahm -- all essentially grew into professionals at major clubs and were quickly moulded into full-backs.
It's difficult to imagine that, had Alaba broken through at Stuttgart, for example, he would have been deployed at left-back -- he would have been made the side's focal point instead. Sure enough, he regularly plays in central midfield for Austria but almost always at full-back for Bayern. Daniel Alves is another example: when surrounded by the world's best attacking options at Barcelona, he was always a right-back, but Sevilla and Juventus often used him on the right of midfield, where his attacking influence was greater.
To return to Carragher's point, it's relatively rare to see "failed centre-backs" at full-back these days at the top level. Tony Pulis' West Bromwich Albion are a notable exception -- indeed, his full-backs are actually very competent centre-backs as Pulis is simply determined to use as much height as possible in defence -- but the vast majority of full-backs are dynamic, attack-minded, wannabe wingers.
So what, then, is the modern attacking full-back all about?
Essentially they are the closest thing football boasts to an all-rounder. Despite some able efforts from the likes of Arturo Vidal, the genuine box-to-box midfielder is largely a thing of the past and their duties have essentially been outsourced to the full-back. The full-back is now the player who doesn't truly excel at anything -- if he did, he'd be deployed elsewhere -- but also someone who cannot have a particular weakness.
The attacking full-back, after all, must boast speed and stamina to charge up and down the line constantly. He also is expected to be composed and precise enough to deliver a steady stream of crosses in the final third. Yet he's also crucial in a defensive sense and must be capable of outwitting opposition wingers in one-against-one battles near the touchline.
The combination of these three major prerequisites, none of which naturally has any relation to another, means the role of the full-back is incredibly demanding. Yet they are, in general, often the weakest players on the pitch.
The concept of the attacking full-back is nothing new: you can find examples from the middle of the 20th century, and it's now been nearly two decades since managers realised full-backs, playing in 4-4-2 systems, were often those with the most space in front of them and therefore started to field attack-minded players there. Yet the speed of football has increased astonishingly over recent decades, and particularly over the last three or so seasons, when an obsession with possession has given way to a frantic, fast-paced pressing style.
It means the athletic requirements of full-backs are greater than ever; it's also notable that Walker has established himself as the Premier League's best right-back almost solely because of his physical, rather than technical, attributes.
Ultimately, £50m appears an extraordinary amount of money to pay for Walker, yet ultimately this is a case of supply and demand: there are very few full-backs who have proved capable of performing at such high intensity in the Premier League. The acquisition of Danilo is interesting: although he was underwhelming at Real Madrid, he demonstrated enough quality at Porto to suggest he boasts the right raw qualities. Guardiola might work wonders with him.
The signing of Mendy is considerably more exciting. After three promising seasons at Marseille and one truly outstanding campaign with Monaco, Mendy has established himself as one of the most complete full-backs around. He is capable of defending solidly and has the necessary dynamism to get up and down the line for the entire game, and it's particularly noticeable that he is also extremely intelligent in the final third.
Whereas other full-backs simply get underneath the ball and loft hopeful crosses into the box, it's notable that Mendy is calm and composed when he enters the final third, often picking up good low balls into the six-yard box or driving clever cutbacks toward the edge of the box. That presence of mind remains relatively rare in full-backs.
Of course, the widespread shift toward three-man defences last season means there's a further divide between defensive-minded and attack-minded full-backs. The likes of Cesar Azpilicueta and Nacho Monreal were converted into wide centre-backs, whereas Marcos Alonso and Hector Bellerin became rampaging wing-backs. Guardiola may well field a three-man defence at points this season, but it probably will be in a fluid system in which he switches between a three and a four. This means his full-backs must also be extremely good in a tactical sense, too.
It remains to be seen whether City's new full-backs can help them win the title. If City do triumph, however, it's unlikely they'll be the stars. Despite their huge evolution, the full-backs will still very much play a background role.