IMO there's never a good time. He can splash or become a bust.
If he splashes, this team will get better, if he doesn't, don't big deal, they could send him down again, he can work harder, adjust and come back eventually at some point in the future. I do not see a big deal.
OTOH If he is that fragile mentally, then he is not a mlb caliber player. He will have up/downs, just like everybody else. Also, he is no that young. Time is ticking.