Giving minutes doesn't necessarily equal developing and nurturing young talent to improve. You will develop and gain experience from playing, sure, but that's considering you are in a positive environment with a manager helping you grow. I haven't seen Mourinho help a player improve his game in recent times, and I can't really think of a young player ever but I could be wrong. In my opinion, he does better with players who already have experience and he can steer them to his system and tactics easier. Youth players have so much to learn, it's a frustrating process that I don't think he's very interested in.
Maybe Mou will surprise me though and we'll see Rashford and Martial coming through this third season with new tricks up their sleeves. If they get minutes but their game isn't changing, then he can give as many minutes to youngsters as he wants, he's clearly not reaching and developing them.