It would be benefit that you power train the player which in good form in match at young age. Those are in bad form even though they are young enough to get the quick skill point, never really perform in the match would be big loss for token spending on Green pack and wasting time on clicking training menu. So it could be the wise that you try to play him on the field with most use formation and position will be use on the player and he is performing at you team then you should invest him with green packs. I got some 18 y/0 5* player once he is Vietnamese I guess. Not really perform so many way i try to put him on field and at the end i sell him out again. So before you buy a player do take note that some country man may not perform well in your team. I think that's why alot of people chasing England player may be they easier to adapt anywhere IMO.

Cheers guys hope i dont spoil your mode of training.