well, you should know all your player will reduce 20% when you are leveling up to next league level,

your player don't get weaker, it just an indicator according to the league level standard.

just keep your promising young player and the strong old player

my suggestion to you for the player you want to keep:

1) keep train and use rest pack on your young promising player everyday, (no need to use token to buy the rest pack, just obtain from video everyday, but if you want to buy the rest pack using token, its still up to you)
2) train the other player also, but don't use rest pack, its a waste, just train them until they will get enough rest before next match.