I think I might have said that many times in this thread, but I'll say it once more....
When we play with a forward 3 and Valencia as RB, the RW which is usually Mata or Mkhi tucks inside becoming a roaming #10 playmaker while Valencia provides all the width down that side. The LW on the other hand is playing ahead of a defensive fullback (whether that is Darmian or Blind) and has to provide all the width. Otherwise our shape becomes too narrow and forwards are falling into each other.
Thus, when Martial plays on the LW his responsibility is to mainly hug the line, provide width and track the full back. He doesn't have the same freedom Mkhi and Mata enjoy. He is not "banned" from moving centrally per se, but he has to be very aware not to drag defenders centrally with him and clutter the space around the main striker or the #10. It's an extra thing he has to keep in mind and be aware of.
What is described above is typical wing play. Martial does not have it in him. Maybe it's a training issue, maybe it's an intelligence issue, maybe it's simply an experience issue... I don't know. But the fact is, when he has the freedom of the park to make runs, he makes them. We saw it today and we saw it most of last season. He can run at players and link up well with the other attackers if he's not saddled with the responsibility of maintaining a shape for the team. When he has to play as winger, he seems very indecisive with his movement and reading of the game. He can't make up his mind when to make a run and when to sit on the line to simply drag the right back away from the centre.
In other words, the lad is a forward and not a winger. When he's part of a mobile attack where positions can be interchanged freely and he's been given the freedom of movement like he was today, he will perform better for us overall. Of course there will be games when teams will park the bus, like the WBA game, where there's generally not enough space for him to move. Of course he'll have shit games too, like vs Anderlecht, there doesn't always have to be an excuse for why he's had a bad game. Bad games happen.
But overall, the more he plays like a forward and the less he plays like winger the better he will perform for us on the whole. That to me, is more than clear.