By now I've studied nearly every aspect of Messi's game, down to a touch-by-touch level: his shooting and scoring production; where he shoots from; how often he sets up his own shots; what kind of kicks he uses to make those shots; his ability to take on defenders; how accurate his passes are; the kind of passes he makes; how often he creates scoring chances; how often those chances lead to goals; even how his defensive playmaking compares to other high-volume shooters.

Yet none of that really addresses the main potential confounder the author acknowledges: most measurable aspects of a player's game --not only (assisted) goals but also even touch-by-touch, more seemingly 'individual' skill measures such as passing success-- are arguably heavily conditioned on the ability of the rest of the team. (Today, for instance, Messi didn't score against Switzerland, and the team was goalless for 117 mins until a tepid 1-0 in extra time (edit: but assisted by Messi! my apologies)... go figure).

A 'matched' design would have been much more appropriate, but I realize it's hard to do sound data analysis on a journalist's deadline.