Defenders are not portable between defensive systems

Recruitment keeps buying centre backs on aggregate defensive numbers, then dropping them into a shape that asks for the opposite skills.

A club watches a defender dominate for two seasons in a side that sits deep. Wins everything in the air, blocks constantly, makes an enormous number of clearances, and the numbers look like a wall. They buy him for a team that defends with a high line. By November he is being described as slow, and he was always slow, and nobody thought to care.

The two jobs share a name and very little else. In a low block the defender is dealing with crosses and shots and bodies in a crowded box, facing his own goal, with support everywhere. The qualities are aerial dominance, physical strength and the concentration to keep doing it for ninety minutes. In a high line he spends the game in fifty yards of open grass, turning, running backwards, judging whether to step up and hold an offside or drop and cover, usually alone against a fast forward.

Recovery pace and decision-making dominate the second job to the point where they overwhelm everything else. A defender who is quick, brave about stepping, and comfortable defending in space can be mediocre in the air and thrive. The reverse is not survivable.

The problem is that defensive statistics are produced by the system, not the player, and they read backwards. A defender making many clearances and blocks is usually in a team that is under sustained pressure. Move him to a dominant side and the volume collapses, because there is nothing to clear, and everything now depends on how he handles four or five isolated moments a game, which is exactly the thing his old numbers never measured.

The clubs that recruit well appear to have accepted this and scout the specific situation instead. How does he defend a ball played in behind. What does he do when the winger drives at him with space either side. Does he step with conviction or half-step, which is the worst option and plays everybody onside.

The counterargument is that a good coach can teach the rest, and there is something in it. A young defender can learn a line. He cannot learn to be fast.

So when a signing looks like a disaster within six months, it is often not scouting failure in the usual sense. Somebody read numbers that described a team and thought they described a man.