It would have been much easier to just write "I think Mata is pretty average". Your whole post seems to dress your opinion up as a series of facts, but these are all opinions made on the basis of you thinking he's average (you say "not true" several times as if you've proven something, when in actual fact you go on to give your negative opinion as to why he's average or why his qualities should be underplayed).
I don't think his qualities should be underplayed (he's a brilliant finisher, final passer, and set-piece taker - and that's great). I just don't think his qualities (when balanced with his shortcomings) make him a top player.
As for the whole 'it's your opinion stuff' - that's fine. If it's my opinion, and it's so wrong, then prove me wrong on my points. Telling me it's just my opinion doesn't count as an argument.
In a few examples you have used the facts of his career and worked backwards to reach a "factual" conclusion, which is actually just you opinion. Examples include: Mourinho sold him (fact) because he isn't brilliant at dribbling/carrying the ball and that's Mourinho's preference (your opinion). Mourinho sold him (fact) not only because he is poor at defending as Mourinho would compensate for that (your opinion). He was great at Chelsea (fact) because they played counter attacking football to a greater extent and he had no defensive responsibilities (your opinion).
You can call it my opinion, or you can look at his previous sides (and current side) and look at the players he has used throughout his managerial career. Carrying the ball in transition is a Mourinho preference, based on this evidence.
Again, evidence (Deco, Sneijder, Ozil) shows that Mourinho will not drop a player in that mould due only to lack of defensive nous. There is more to it, otherwise those guys would have been sold/benched like Mata. Unless you want to argue that each and every one of them was forced into the side by each club owner/chairman.
Anyone could do this with any player. "David Silva - he's a great player. Not true. How many goals has he scored throughout his career as an attacker".
You could say that for Silva. But it's daft. It's obviously a poor example. It's not even similar to what I've said about Mata.
Most fans tend to look at all the excellent things their best players do contribute to the team, rather than the things they don't contribute or things they aren't expected to contribute. In Mata's case it's quite easy: a quick look at his direct contributions to our goal tally will show you why he's one of our best players. Somebody actually showed stats that when Mata plays, the team scores less.
The things that Mata doesn't contribute (poor defensively, lack of ability to dribble players, to shield the ball, to turn a man, to keep hold of the ball...) are all important for a top player (with exception for the first one). How anyone can claim he isn't expected to contribute these things, I don't know. If he's not expected to contribute these things then the team is probably not very good. Coincidentally, Chelsea weren't very good.
You've told me many times that it's all just my opinion. So I can only assume you think my opinion is rubbish, so it shouldn't be hard to actually address the points and prove them wrong.