I have a different perspective on this. Jose gave a candid interview over the summer about Rashford and why he likes him so much. He feels that Marcus as a player can be put under pressure and will take it on his chin, and perform the way Jose wants him too. He doesn't want 'soft players'. For me, it looked like Jose didn't take as quickly to Martial as he didn't see the same response.
This season, Martial has come back to the club and has been putting some brilliant performances in pre-season. (Probably better than what he did all of last season). Jose would have noticed that. A public spat with a wife (involving a child), a new GF and tough club environment can be very tough for a 19 year old.
Now - with a rest - a few things behind him, Martial seems to have refocused. Jose may well have kept him on the bench against Real knowing fully well that he will start on the weekend and to see how he responds to that. He will need to be able to rotate between Mata, Martial, Mhiki, and Rashford this season to keep everyone fresh. So, it could be more of a test of attitude than anything else.
I believe that if Marital went back to training pitch without letting his shoulder drop after the RM game, there is big chance he will start today.