His off the ball movement is non-existent and will hold him back as a player. He would much rather get the ball to feet on the touchline and then run at the RB, rather than make an off the ball movement in behind or a curved run.
There is normally a few reasons as to why this is, firstly he loves dribbling and is always looking to beat his man, it's the first thought on his mind, and he knows he will get the ball by occupying this position to than attack the full-back. Secondly, he doesn't feel he will get the ball as much making them runs which is understandable, you have to sacrifice and gamble alot which Mkhi does really well, you won't get the ball every time, it may not be played, may get cut out or the full-back may stop your run but you have to do it for the one time it works because then you are in behind and it's a great chance at goal.
Probably at the moment is Martial is getting the ball to deep and can't really square the full-back up one-on-one, he is getting doubled marked or the wingers are backtracking which basically means he's running into blind alleys.