I watch as much as Utd as i can (they are on TV 1,716 times a week so i'm bound to catch them at least once lol) but even he's other numbers are down (goal the exception as you rightfully pointed out).
His average key passes at Chelsea:
11/12 - 3
12/13 - 2.8
13/14 - 2.3
With Utd:
13/14 - 1.9
14/15 - 1.5
15/16 - 1.4
16/17 - 1.9
Its good to see his goal stats up at Utd however i'd argue he's far far better creating, maybe its the way Utd play i'm not sure.
My main gripe however isn't actually at Mata its mainly Mourinho. For the first time i can look at him from the outside an unbiast view if you will and its embarrassing the stuff he continues to come out with and i see some of Utd's fans comments regarding him and the teams form etc and i remember me and other Chelsea fans saying the same things! The mans attacking players publicly in season 1, questioning players commitment despite them being injured and contradicting himself time and time again.