In '12-13 Chelsea played a load of teams in the Europa League while all their games the following season were in the champions league. They also had one of the worst teams in the competition in Norddsjelland in their group and who only ended up 2 goals shy of the worst goals against record in the history of the CL. Same with the FA cup and the league cup. A lot depends on the draw you get and the teams you face. I mean Chelsea gorged themselves to 5 goals against a defensive pairing of Scott Wooton and Michael Keane when they played us.
You can't really compare their scoring records from different competitions, especially when one of them is a 2nd tier event. The league comparison either seems the fairest way to look at things even if it's not perfect.
I pointed out why I said that however you completely ignored that part of the post for some reason. I also don't understand why your including his goals when we're discussing his creative influence.
For a start, only looking at assists is not the best way to judge creativity although it does provide a good indication. The assists stat includes set pieces and 5 yard passes where all the work is done by the person receiving the ball. Mata has 33 assists from open play in 135 games at Chelsea. That's a much better indicator of how much he creates than lumping in all his set pieces and goals scored.
Di Maria is miles out in front as far as creativity goes so I won't even bring him up. Let's look at Rooney. From deeper positions, he has more assists from open play, more chances created, more key passes per game and more through balls played per game. But, really, you don't even need stats to know Mata has struggled to create. It's been quite obvious on the pitch. His passes are more often than not safe passes. More than 50% of his passes this season have gone backwards which says a lot.
But why do that? What has Mata done for us to trust him in that role? Herrera scores goals, assists goals and offers a lot more defensively. On the basis of this season I'd much rather have him in there than Mata.
I understand your point about playing in the Europa league but even if you ignore those games their record is still way better than the next season.
I've never argued shifting Di Maria out for Mata so not sure why you've brought that up. Putting one player in his proper position doesn't necessitate marginalizing another. I'm also unsure as to why you're bringing up Rooney's record from deeper positions, I'm talking about Mata's record as a #10, not stuck in midfield like he has been for chunks of this season.
On the assists from open play issue, I have it him down for I think 37, but going on your stats - 33 in 135 is 0.24 assists a game from open play. Including set pieces his record is 55 in 135 which is 0.40 assists a game. For comparison -
Rooney - 143 in 646 (entire career stats according to his website) = 0.22 a game including set pieces.
Giggs - 162 in 632 Premier League games = 0.26 a game including set pieces.
Van Persie - 38 in 196 since 2009 = 0.19 a game from open play.
Messi - 111 in 299 games since 2009 = 0.37 a game including set pieces
Suarez - 35 in 143 since 2009 = 0.24 a game.
With that in mind, you seem to have a pretty unrealistic concept of how many assists a creative player should be getting, from open play or otherwise. Either way Mata is up there when it comes to productivity from open play. Even then the whole thing about 'defence-splitting passes' being a marker of creativity is entirely subjective, creativity is about what you create, surely. I find it especially strange questioning Mata's record for Chelsea whilst citing Herrera as being more creative. I've followed Herrera since his time at Athletic and I like him as a player but he's never been particularly productive or creative. His record last season where he played exclusively as a #10 in a good Athletic team was 4 open play assists in 33 games (0.12), even including this season he's only assisted 19 goals from open play in his last 174 games (0.11 a game). Mata's record even disregarding his contribution from set pieces is superior to every member of our squad barring Di Maria. I mentioned goals as well because the fact that he gets as many assists as he does whilst also being a prolific goalscorer is part of what makes him such a good player.
As I've said before, Mata's done plenty to prove himself to us as a #10. In the games where's he's actually played there he's been very productive and consistently been one of our best players. As #10 he's assisted 7 from open play in 21 starts in his United career (0.33) whilst also scoring 8 goals, which is a fantastic return. His problem this season isn't that he's been bad, it's that he's played less than half his minutes this season in his natural position.