Whilst that is a fair post, it must be noted that both he and Rashford, for all their flaws and learning points, are having to have right footed, mediocre left backs as their support.. and to make it worse, those average full backs are not instructed to provide width on the overlap and instead they tend to underlap.
Neymar... Alba/Marcelo.. now Kurzawa.
Hazard.. Marcos Alonso
Sanchez.. Monreal/Kolasnic
Now not saying these guys have had top tier full back support, but in general they've had pretty attack minded left footed full backs behind them, which gives them more freedom to cut in. Whereas we have very defensive full backs and no left footers in the squad.
If Shaw had lived up to his potential, it would definitely bring on Martial and Rashfords game leaps and bounds.