I get this. And I suspect it is also a factor in selection, which also makes it uneven. Martial can never change the fact he's not a Manc!
I believe Rashford is the chosen one. Jose has a local talent of genuine ability and a real chance to make it, and also given him a perfect testimony of youth development. I believe he will be afforded every chance to become a first teamer, whereas Martial will only get that chance if Rashford first fails to take his. I suspect that, as long as Rashford performs reasonably well, Martial can never be a regular.
The likelihood is that Rashford will eventually pull away from Martial anyway. Martial has done incredibly well to hang in there so far, and this is why I suspect Jose has been more objective on the topic in press conferences. It couldn't have been expected he would make such a statement with such little cameos, and then there would be no debate at all. The chances are, unless his sub appearances start earning him regular starts, he will inevitably fall behind anyway, as it is unrealistic to keep expecting him to do more in 20 mins than Rashford does in 70. Sooner or later, all the justification needed will be provided for Martial to be second choice. It will be on the basis 'Rashford is playing better'. Martial has done extremely well for that not to be the case so far given the circumstances.