Mourinho’s argument would be that Mata is not dynamic enough, with the manager preferring more direct attacking, with players running at defenders, rather than the Spaniard’s scheming playmaking.
One thing going for Mata is Mourinho’s preference for inside-out wingers - hugging the touchline, cutting in onto their natural foot.
Mata is the only player at the club who can perform this role on the right - with him being naturally left-footed - but he doesn’t perform it in the same blistering, run-at-the-full-back way that Hazard does.
Last season, Mata, Hazard and Oscar rotated at free-will between the three attacking midfield positions. This won’t happen under Mourinho.
Oscar’s performances in the first three matches of the season have seen him seal the spot behind the striker, coming deep in midfield to collect the ball and influencing play higher up the pitch.
The Brazilian, however, screams ‘Mourinho player’. His incredible work-rate, ball retention and creative spark bely his 21 years of age. He’s just as likely to be found helping out Ramires and Frank Lampard if the opposition have the ball as he is laying on an assist or scoring from 25 yards.
This is where Mourinho’s opinion of Mata wanes. He’s not a lazy player, far from it, but he doesn’t track back well enough to function in a Mourinho team.
Hazard is similar at times, and the manager said as much in his press conference ahead of the United game, but the Belgian is much more explosive than Mata - so he gets a pass, for now.
Mourinho wants lightning-fast counter-attacks, delivering teams a knockout blow on the break. Mata’s slow, scheming brand of play-making isn’t compatible with that. He’s not slow, but not nearly quick enough to play that way.
He can pull off incredible feats of skill, lay on golden opportunities, and score goals from anywhere, but that isn’t enough for the boss.
In his first spell at Chelsea, Mourinho frequently dug out Joe Cole for his lack of defensive work and his tendency to aim for the spectacular rather than contribute to an attack meaningfully.
He made Cole a much better player, but was still never a massive fan of the England winger. Mata has been such a key player for Chelsea in the last two seasons - he played 64 games for the club last term - but the feeling is he will be reduced to a bit-part role.
But will he really accept that? As someone who has clearly been in the top five Premier League players over the past 18 months, will he be happy to play one game in three, competing with playing time with Schurrle, De Bruyne and new boy Willian?
The difference in Chelsea’s results with and without him are clear. In the six matches in which Mata hasn’t featured, the Blues have won just two, scoring seven goals in total.
It’s unlikely there’s an attitude problem. Mata is widely considered to be one of the model professionals of the club, whose cheery demeanour has been clear throughout.
It seems unthinkable that he’ll be carted out with the likes of Yossi Benayoun and Marko Marin, but there’s also reason to see how he doesn’t fit Mourinho’s plan.
The ‘Special One’ could find a slight sticking point in his harmonious marraige back at Chelsea if the situation isn’t resolved positively.
Blues fans know - as Mourinho will - just how good Mata can be. Let’s just hope the manager’s plans give him added defensive and creative attributes to make him even better - he’ll have that Player of the Year award sewn up for years.