Here are some thoughts...
Mata is incredibly skilled technically. He is also a slow player, in that I mean he takes his time on the ball. Under LvG that was great. Unfortunately for Mata not much scoring happened but when it did Mata was usually contributing. Now you want to tell me Mata is average, i would argue average in a very substandard attack makes him pretty good.
The issue under Jose is he likes quicker attacks. No dawdling on the ball. Get it up field quickly and exploit. Also Jose likes his players to work on Defense. Mata has totally stepped up his game in that department as noted here...
http://www.telegraph.co.uk/sport/fo...anchester-Uniteds-hardest-working-player.html
So in theory it really comes down to the question of Mata playing the ball quickly enough on offense. The reality is Rooney and him being nearly un-droppable.
I would love to see Pogba behind Mata with Martial, Miki, Zlatan and or Rashford around him. I think you would then see some real quality.
Now against the bigs on the European Continent where Jose has the team sit back and counter, I do not think Mata will get much time. I do think Mata could help unlock some EPL clubs parking the bus. Oh well we will see.