Personally I don't see the point in willing him to fail or taking pleasure out of a bad game for him or dips in form if/when they come. He's 18, so off games, tactical/positional deficiencies will show because he's still learning, he will also probably have some dips in form to overcome unless he's really lucky.
I think that's the benefit to Peps approach of easing them in, perhaps he's guarding against some of the above. Some players come from nowhere after being given a chance and surprise even the coaching staff but those are the exception. Putting a promising player in at the deep end, unprepared and letting them get completely lost in a game could set a player back a few years, if they are confidence players. There will still be a learning curve but at least tactically you know Pep will have had them well drilled for years before they make the step up. If they still don't get it after having years of Pep and the world class players in the first team to learn from, then perhaps it wasn't meant to be. Maybe you could say in that instance that maybe a different manager and system would suit them better, maybe they aren't quite top level player but can still have good careers with everything they have managed to take on board. I don't think there is a one solution fits all answer to bringing through top players, if there were, everyone would already be doing it.