Not only does he play every single game, he's also the ONLY player in LvG's side that's afforded the freedom to roam away from his position. Rooney, Martial, Lingard, Memphis, Herrera, etc all stick to their positions, more or less. Mata, on the other hand, spends more time in the middle than he does out wide.
He gets special treatment and a guaranteed starting berth. I'd say LvG must like him!
Hell, at the start of 2014/15, we played a 3-5-2 largely to accommodate his roaming in midfield (after Kagawa had failed in the role). Mata is the perfect 'philosophy' player. Impeccable in possession, but still with decent output. Compare him to Di Maria and you'll see that those are the only areas where Mata is superior, so it stands to reason that they're also why ADM was shifted about but Mata has had a special role for the entirety of LvG's reign.
Mata might not be a 'special' player in terms of overall quality, but he certainly is 'special' in terms of what LvG values with his football.