Claudius Typically about a dozen games is enough to have a clear pattern either way. At worst you wouldn’t be able to prove that he makes the defense better, but you won’t prove that he makes the defense worse statistically.
You could try if you did more methodological and modelling legwork. But there's a need to account for many factors other than Zinchenko's presence or absence. For example: which player replaced him, strength of the opposition, the rest of the selection, some sort of "familiarity effect" (any team gets used to covering for players with specific frailties over a stretch of matches) … it goes on.
That's only the stuff that is fairly obvious.
There aren't any fan stats sites or services that publish data or models on that level?
It's fair to say a crude "xGA with versus xGA without [player]" stat won't be trusted if it lacks a corresponding narrative from basic observations … or in supporter terms, the eye test. I'd tend to be just as interested in the eye test without the stats, though both have to be in question given such a granular claim.
Without all that, the main use of such a stat won't be to establish a claim, but to call other claims into question, as you're doing above.
As far as any "Zinchenko effect" is concerned, there's a coarser-grained inference this season that the drop in our xGA has been linked to pressing and ball retention high up the pitch. Perhaps Zinchenko's contribution to that during his Arsenal career has outweighed his higher profile defensive failures. Or perhaps our available backup LB has just been a bit shit for one reason or another (inexperience, unfamiliarity, traits etc) during Zinchenko's absences.
