I wouldn't say it is player quallity, but rather rating or, even better competitiveness. Stars show how competitive player is at your level. In top eleven player can't lose quallity, only improve it, but can lose rating/stars.
The best way to explain that is in loan system in real football. For example, Liverpool's young winger Lazar Marković was sent on loan because he wasn't enough competitive for BPL in that moment. At least that is what their former manager thought. So, he moved to Fenerbahce where he is more competitive for 1. Turkish league. That doesn't mean his quallity dropped just because he moved to weaker league. There he improved his skill and current Liverpool's manager claims he will play again in the BPL after loan ends. There are many cases like this one.