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With the reintroduction of three-man central defensive systems in world football, wing-backs are becoming more common.
When you run a system with wing-backs, it pretty much lives and dies with them. If they make a telling impact, they can run riot and help you destroy a game. If they can't find a rhythm, you might as well surrender from the start.
These are some of the most well-rounded players in the game and have to be superb on both technical and physical levels.
One minute, you're busting a lung to get forward. The next, you're busting the other getting back to defend.
Between all that running, you need to pass, cross, shoot, tackle, mark and (possibly) take set pieces.
The real question is, who'd be a wing-back?
Classic example: Cafu, Roberto Carlos
Current example: Mauricio Isla, Sime Vrsaljko