De Gea's flaws as a player are irrelevant in this case. The same with Mata.
Because what they're referring to seems to be basic professionalism. There are obviously players who haven't been meeting the required standard behind the scenes, who have have a bad attitude, who don't want to be here, who are leaking shit to the press and who are creating a toxic atmosphere.
And the likes of De Gea and Mata are entitled to call out that lack of character and desire to be here because flawed though they are themselves as footballers, they at least have the credit of having lasted a long time at the top level while being consistently regarded as good pros and teammates. It sounds like the people they're referring to wouldn't be able to say the same.