I wouldn't read too much into that.
I think we all know our league position would/should be about 5 to 7 points better, and isn't due mainly to some abysmal finishing & in a couple of instances, brilliant goal-keeping. Rashford started in some of those matches and may have missed a couple of chances but there have been several other guilty parties on that account, not just him.
As for when he starts, it looks like Mourinho seems to think both Rashford & Martial are future candidates for the CF/main striker role, and many here on the caf would agree with that?
To that end Mourinho started Martial up top against Hull for example before he switched him to the left side where he prospered (as he clearly struggled as the spearhead in the first 45) and we saw the same against Leicester only this time it was Rashford up front (to no avail) before being sent back to the wing.
I don't agree with those saying these kids can't or shouldn't be playing on the wing; or that they are so much better as a central striker. They're both young, can run like feck and both need as many minutes on the park as possible. I don't care where they play as long as its up front of course and as long as they work hard, learn from their mistakes and from the wealth of experience around them.
Of the two, I get the impression Rashford wants it more and tries harder in games and probably in practice. And that's why he's getting a few more chances than Martial.