Many young players have talent, but it's how that player applies himself and uses that talent.
And an output of 4 league goals for the whole season, suggests that he has A LOT of developing to do.
I just don't think that Jose is the right man to get the most out of Martial. Martial needs to be mollycoddled (Shaw is the same) and Jose is not the sort of manager who does this, which is why Jose tends to do better when coaching grown men, rather than young players. Rashford is the exception, but by all accounts, Rashford's attitude, behaviour and commitment seems to be more mature than his age would suggest.
Assuming Perisic is a hardworking player who will 'run through walls' for Jose, Jose would play him in every game, over Martial (who doesn't seem to care to impress Jose and certainly won't be running through any walls for his manager).
I know it sucks that Martial has gone backwards, but that's what happened last season and tbh, I expected younger players to struggle under Jose, but I will take that, because Jose will deliver trophies and trophies are what MUFC are all about.