It's a big difference going against De Sciglio and Kyle Walker, and then going against Ashley fecking Young. Plus Sane had more space than Martial ever did in that game, especially since we were throwing people forward at that point.
This is the same thing that teams have been doing to Tony since midway through his first year with us. Double up on him every time he gets the ball on the left, and allow space to be had on the right side because they know we don't have a player that will punish them accordingly. Young operates in acres of space almost every game that Martial plays because the entire defense is shifted towards one side of the field, but rarely does he put any decent delivery into the box, so teams let him have it. Plus, we haven't had a decent RW in ages to solve the problem either. The day we decide to actually buy one (fingers crossed), is the day you'll see Martial back to skinning his defender regularly and having more space. Right now teams just disregard the entire right wing and let us mindlessly cross it in from there, knowing nothing will come of it. It drives me crazy though when fans complain about Martial not taking on his man, when he has two guys standing in front of them, because if he did and screwed up people would be bashing him for being selfish and trying to do it all himself.