Therein lies the flaw with your argument. Proven? When? One solid season in the Bundesliga a great player does not make. I'm not going to count his performances in the Ukranian League, because it's weak and his team were like SS3 Goku fighting against a bunch of Krillins.
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Why do you only look at stats? Judging him by stats is so misleading. A player can excel in a few games per year, thus padding their stats.
Does Costa contribute against the big guns? Does he have an impact in important matches?
Bayern score a $#@!ton of goals against the likes of Hamburg and FC Carl Zeiss for example, so it could mean that your beloved Costa had 4 goals and 4 assists in those two games alone, which would then imply 3 goals and 5 assists for the rest of the entire season. Not so impressive now, is it?
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I wish we'd do that for once, without selling our best players.
A Sanchez + Verratti Mercato would have been the dream for us. Alas, it's not meant to, and probably never will, be.