Martial and Rashford, both of them just a child in current world of football...they are not fully developed. Inconsistency is their friend. Technically and Mentally, they must apply a spirit within theirs to improve.
However, the guidance from senior players is needed. In this case, based on theory only, the task should be done buy one of Rooney or Ibrahimovic. Sharing the knowledge of the game and the mentality, the risk, the benefit, and so on. But that's only a theory, because we are not in CM/FM game.
All players hired expected to become an adult both as a player and as a human....and it is not easy specifically for young player like Martial and Rashford.
Like I said in other thread, Martial will come good, not a world beater a la Henry, Ribery, or Benzema, but will be better than Ben Arfa, NAsri, and Valbuena...If...his head and his heart always wide open for improvements (including Critic).
Just my 2 cents.