Is this really all that uncommon though? There has been countless examples of players who have started off well and then had underwhelming careers. Especially when Martial seemingly (according to his fans on here, at least) can only flourish in very specific setups.
As a player, he needs to learn to adapt. Granted, he might go to another more attacking team and do well, but not every manager he's going to play under during his career is going to be possession based, constantly giving the ball into his feet for him to run at defenders.
For example, that's not how France plays and until he improves those aspects of his games, I don't see him playing much for his NT either.
Mourinho probably isn't the best manager to get the most out of his ability and if he's really that intent on going, let him leave. I think he's a really good talent, but not in the top tier of youngsters around there.