"Juan Mata can't play in 4-3-3." This isn't some manufactured strawman -- it's a genuine opinion held by many Chelsea fans, worried that a return to a three man midfield could spell the end for Stamford Bridge's favourite Spaniard. And it's also demonstrably false.
While it's true that Mata is one of the world's best pure number tens, a position that doesn't exist in 4-3-3, to assume that that will prevent him from playing well is to miss the footballing forest for the tactical trees. The argument also merrily discards the mountains of evidence to the contrary, for unlike most debates regarding whether or not a player suits a given position, we've seen plenty of Mata in an inside forward role.
In Andre Villas-Boas' 4-3-3, the only two players who consistently performed at anything like Chelsea standards were Ramires, cast as a roving central midfielder, and Mata, who played as a left forward. Mata faded in the second half of the season as Roberto di Matteo moved him to the centre, but in the early stages of last season he once again found a role -- and enormous success -- in a wide position. Granted, Mata had the leeway to come central, but the number ten position he supposedly needs to thrive was then occupied by Oscar.
It's only under Rafa Benitez that Mata's really shone in the middle, and while he showed he can be an elite central playmaker, that doesn't exactly mean he's incapable of making a serious impact from wide, especially when he's given the freedom to move around and interchange.
It's probably true that Mata is more suited to a 4-2-3-1, but he's a world-class player who is completely capable of adapting his game and excelling in slightly sub-optimal conditions. The assertion that he'd struggle as a wide player in a 4-3-3 is not only an insult to anyone who's watched Chelsea closely over the past few years, it's an insult to Mata himself.
There are issues, certainly, with Mata as an outside player, most notably his inability to protect the fullbacks in any meaningful way. But this is a flaw in his overall game, and manifests itself (perhaps with greater effect) in 4-2-3-1, where the abysmal performances of Chelsea's pivot midfielders were at least partially attributable to the non-existent defensive ability of the man playing in front of them.
In football, we define players by their position too often. What we should be doing instead is looking at players' individual strengths and weaknesses as well as how they might interact with one another, then optimising from there. That doesn't mean a total football free-for-all, of course -- some players are obviously totally miscast in certain positions -- but an adaptable, clever attacking midfielder who's a minor defensive liability moving wider into a position he's already played well isn't exactly as dramatic a switch as trying to pretend that Ramires is a possession midfielder.
Can Mata play 4-3-3? Of course he can. Not only does he have the ability to succeed there, he's done it before. Is 4-3-3 the optimal shape for Chelsea? That's a far more difficult question. I think so, but the answer isn't simple by any means, and Jose Mourinho's far better equipped to deal with it than I am.
For me, the Mata question isn't about his position but rather the ability of the team as a whole to keep hold of the ball. As we saw against Barcelona and Bayern Munich last season, Mata's effectiveness isn't a function of where he plays but how often Chelsea can get him the ball. And that implies that the players behind him might matter more than his starting position (he'll roam all over no matter what anyway).
There's an interesting debate to be had about how to get the most out of Mata, but it certainly doesn't start with the assertion that he's purely a 4-2-3-1 player.