Some positions can be rotated freely and centre back is not one of them

Rest is treated as a squad-wide budget when the cost of a change varies enormously depending on where the change is made.

A manager with a fixture on Wednesday and another on Saturday is usually described as making five changes, as though the five are interchangeable units of freshness. They are not. Changing a forward costs almost nothing in coordination terms. Changing one half of a centre back pairing costs a great deal, and changing both is close to negligence in a match that matters.

The reason is that defensive organisation is a shared skill, not an individual one. Two centre backs who have played sixty games together have an unspoken agreement about who steps and who covers, how high the line sits against a particular kind of runner, and what happens when the ball goes over the top. That agreement is not written down and cannot be transferred by a team meeting. Break the pairing and you are not fielding a slightly worse defender. You are fielding a defence that has to make decisions consciously, and conscious decisions are slow.

The same applies to the goalkeeper and his back line, which is the relationship clubs disrupt most casually. A cup goalkeeper comes in with different distribution and a different starting position, and the defenders in front of him spend a half adjusting to a man who claims fewer crosses than the one they know.

Forwards are the opposite. A striker's job is largely individual in the final third, the runs he makes are read by defenders rather than by teammates, and the cost of swapping him is close to the difference in his individual quality. Wide players sit somewhere in the middle, depending on how much the shape asks them to coordinate with the full back behind them.

Which is why any model that predicts rotation as a function of minutes played and days between matches is missing the important variable. It treats the squad as a pool of interchangeable fitness. What a manager is actually deciding is how much accumulated coordination he is willing to spend, and coordination is rebuilt slowly.

If I were building the tool, I would weight each position by how many of its decisions require another player's agreement. Rest the forwards every week if you like. Leave the pairing alone.