You don't need to power train them to keep their rating high if u don't have tokens or packs, unless you are gunning for legendary quality.

What I do and did when I started was for these players, I simply play them every game and do a postmatch match practice just for them. I've had one player from Level 1 to now Level 8 for Real Messy, he started as one of those exp youngsters in lvl 1 and he has been a 4-5 star player, which is good enuff for me.