I think it's as simple as Rashford is English and from the Academy, Jose knows he's expected to make him a key player and unfortunately for Martial the only role Mourinho can give him regular starts in is his position, it feels like no matter what Martial does he will never get the preferential treatment Rashford is afforded, and the fact the #9 role will be occupied for the foreseeable future makes it even harder and he won't play Rashford on the RW because with both on we concede too much control, Mata is playing so much as he's essentially an extra piece of the midfield that helps our possession.