Excellent point, but would you care to explain how you would match subjects in this case? They should be comparable pre-something, but what would that something be?
Perhaps a superficially plausible ignorability assumption would be that, during rapid breakouts from the defensive zone, striker skill would strongly depend on the midfield's ability to open up favorably, which in turn would depend somehow on the defense's overall (relative) skill; so that, viad-separation, striker skill and defense's relative skill would be independent conditional on midfield's skill during breakouts. It would definitely not be trivial!
