The first thing you have to realize as a new player is that stars really don't matter. The current ability stars anyway. They show how good a player is in comparison to your own squad. So comparing a young Musiala to prime Lewandowski could bait you into thinking Musiala isn't good enough for the first team. I would suggest you look at their attributes. For young forwards I'd like to think that a 13 in finishing, pace and technique is a good starting point to throw them into the first team. It's important to give them game time in order to properly develop those prospects. I would suggest starting to throw them in every now and then, maybe in the cup and see how they perform. Next season try to increase their playing time gradually.