Martial started the season as a striker, and over the course of it he has been switched to the left forward position and that's when our system was starting to shape quickly to making him the focal point like Robben in Netherlands. I don't think that's a " very subtle difference " imo. It was clear that the ball is always reaching him on the left flank and he's always the one who creates the chances and you can't watch a game and think he's invisible. That's not like the last season when the ball was always built for Zlatan or down to the right flank for Valencia. The pattern we were attacking with is different and choosing other targets and that made him appears invisible in some portions of our games. Martial needed to cope with him not having the ball all the time but rather asking for it and I think he's doing pretty fine this season,
I don't think him having ups and downs also contradict my point. Martial's off ball movement has been a problem, but if you're focusing the play on him and supplying him with the ball every attack you'll never notice it, but when you switch the attack to other targets and make Martial a part of the system rather than being the focal point of it, he doesn't have the ball all the time and hence his off ball movements problem becomes clearer to be noticed. That's also the part he's improving a bit this season.