Martial and Rashford were both playing much beter before Zlatan arrived at the club and since Zlatan has gone they have picked up their former form back again. I don't think we need Zlatan to help these young players progress.
Also Lukaku has been mighty fine on his own and he doesn't need someone like Zlatan to tell him how to be a striker.
Zlatan is all about Zlatan. And when he can be your star player on the pitch the man has enough confidence and leadership to carry a whole team. But if he needs to play a role from the sidelines, I'am not sure how that ego of his will work out. If he can't be the big guy on the pitch nor the big guy on the side (Mourinho is) than how will Zlatan's ego feel and how long will it take before that starts poisoning the dressing room ? It is one thing being forced to sit on the sideline due to being injurred, it is another matter playing that role when fully fit.
Zlatan thinks himself the king, that is his strenght and kings don't serve, they rule, so a serving job like a coach doesn't strike me as a role for Zlatan.