If a player's work rate is high (usual range is 45-85 at age 18-21, in practice match while training normal) doesn't automatically mean that he is good, nether stars can tell how good he is. My 2-stars free player from youth academy plays much better than 4-star fast trainer (18 tokens), and he didn't play any better as 5-star player. If a young player (fast trainer) has 3 stars and plays good, be sure he is worth to train him hard to 5 or 6 stars, but you will have to test him first in tough games. Otherwise, don't waste your rests... Take into consideration that some players need time to adapt before they start playing well, and some will never play well...