Your right but its about a team harmony, Mata has emerged as a senior and important figure out on the pitch and in the locker room and has performed well in previous big games, not to mention the derby isnt really the right game to start giving players full debuts.
Id like to see it, but I am not a CL winning manager and my head tells me Jose will look at the bigger picture in terms of team performance. When we play, we never play with a regimented 4-2-3-1 our set up under Jose has been far far more fluid and looks a little more like this in attack....
DDG
Bailly--------Blind
Valencia----------Shaw
Fellaini-Pogba
Rooney
Mata-----------Martial
Ibra
With Mata, Rooney, rotating the 10 and wide positions, Width coming from the overlapping full backs with Martial less likely to hug the touchline and cutting inside more with Rooney dropping to the deeper position when he does so. Pogba pushing on from deep when the space opens up. Fellaini the most central player, operating from deep almost exclusively, The back line forming more of a Split 3 depending on which flank bombs on.
Just some small things I have noticed. So far its fluid and working very well, what I found interesting is Miki taking up the Mata position is he tends to come inside and drive from deep more than Mata does. Mata operating in a more traditional wide role and cutting in for the role of No.10 playmaker or spreading the ball quickly, not driving and taking the ball on, in my opinion once the team is more comfortable in creating chances lke this we will see Miki come in.