When a key player is out, you ask how much the team loses. The answer is not "a lot" or "nothing". It is a number built from two things: what the player did and what the replacement will do.
The share-of-xG idea
For each player you measure the share of the team’s chances that pass through him: xGi plus xA over the total. If a creator has 22%, he was the shooter or the passer in 22% of the chances. It is not 22% of the team’s output, but a first upper bound on the damage.
The quality of the replacement
The replacement does not give zero. He gives a fraction of what the starter gave. If he gives 60%, the team loses the other 40% of that share.
What the calculation does not say
- Teammates adjust. The calculation is a rough estimate.
- The share comes from past stats with him in the lineup.
- The coach may change the system, not just the face.
- Losing a centre-back or a goalkeeper affects the opponent’s xG, not your own.
In practice, a key player also has value that xG does not show, such as pulling defenders toward him and opening space for others. The xG share does not measure that, so we treat it as a starting point and not a final number.
How to judge the replacement
There is no magic number. You look at what the replacement did in the minutes he played, where he comes from, and whether he has played that role before. With few minutes the estimate is uncertain, and the right move is to try several scenarios, as in the chart.
- Replacement 40%: a youngster with no real sample.
- Replacement 60%: a teammate in the same position with fewer minutes and a lower level.
- Replacement 80%: a player of almost the same level.