When I see Martial, I don't see a winger. To me he's a striker and he should be playing closer to the goal. The closer he is to the goal, the less he needs to worry about things other than pressing, attacking and scoring goals. If he's playing on the wing, it should be more like Neymar is playing on the wing - a wing forward. He's not the player you ask to run up and down the flank, cover up for the full back or to sprint full speed from a low defensive position to the opposite penalty area. Martial is just not that kind of a player, or person. If you want that, then play Lingard.
So the problem is not the position itself, it's the tactics for him in that position and his own mentality.
The way he's asked to perform now, is way too defensive and tactical for his likening, and for my likening as well. I'm sure he's not comfortable or confident with the defensive duties he's asked to do.
The same goes for Rashford. You don't play players like that in a defensive position, hindering them from performing at their best, where their contribution is greater than others.