I don't see what is so difficult to comprehend here. Wenger's stance on the transfer market is not new; he has on numerous occasions said that HE feels there should not be a January transfer window at all, and that the summer window should be shut before the new season starts. In other words - you start the season with a squad, and you finish the season with the same squad. In this example with Mata he points to another thing wrong with the RULES, not with this transfer specifically; that it shouldn't be possible for clubs to strengthen opponents' squads in the hope they take points off their rivals when they themselves have played them twice.
On a personal note, I agree with Wenger on this, but I'd like even stricter rules; I'd like the same rules applied to players to apply for managers as well. The shortsightedness of clubs is really not doing the game any favors.
