For EJM, it's all about adding to the repertoire of plays that he can execute cleanly. It takes time. He has to execute these plays during a game, against different coverages, different skilled defensive players, different Dline rushes...on and on. He has to learn "on the job", and we have to watch him struggle with it.
Hackett also has to pick the right plays, at the right time, to allow EJM to learn on the job, but not hurt the team as he is getting the experience. Send in a play that EJM hasn't mastered, and it's a gamble that can blow a drive. Send in the plays that he has shown the ability to run cleanly, and it's already on tape, and the defense has prepared to shut it down.