Exactly RJSlow. I recently sold away an 18 year old 5* quality 27 striker cuz he didnt perform good enough and trained pretty slow.
I would prefer to lend him out for the rest of the season to another club so he get experience and games. So once i get him back - his maybe good?

Just as in IRL football u can only borrow a certain limit of players per season. And you decide for how long.. weeks/months or the whole season. Future release fee or simply get him back when the time has passed.

Another idea would be that every team has a youth team where u can send your players to gain experience.