It's not just Zlatan. The team's balance on the left is causing problems too.
Pogba is undroppable (a debate in itself) while the left-back is constantly changing. The 'need' to play Pogba more attacking with Carrick deeper means:
1) Only two out of Mata, Mkhitaryan and Martial can play in attack, and
2) Mourinho would prefer his left winger to be more reliable rather than extravagant (eg. Willian). The winger on the opposite side can have more license because of Herrera and Valencia.
Which is why, as of now, he might prefer the Mkhi-Mata pair over Martial-Mkhi.
But yes, Martial should be playing, and no deeper than the role of left inside forward. This 'striker-as-left-midfielder' stuff is baffling.