fcuk star players, man. They almost got me fired multiple times. Unfortunately, injuries are the only way a star player will accept not starting a game.
Have you tried 'discussing a rest' with them? Sometimes they agree to sit out a week if they are professional personality.
As for goalkeepers, I have two 'backup' GK. I had a first-choice but he wanted to become an 'important player', so I sold him.
If you are giving a contract and you don't have a first-choice, it will be hard to convince him to accept backup status unless his CA is way lower than the others in your team.