In my game I speak to the player, and i tell him in calm tone "unfortunately [player in their position in starting 11] is playing better right now, but you'll get your chance if his form declines" can't remember the exact text. but it has worked every single time with me and this is my 6th season
I never fine unless its a straight red card, and i only warn for 2 yellows, Discipline been always great this way with me
when a player complains to get sold, you tell them calmly also "the bid I received wasn't good enough, I'll accept if I received a bid that match your ability", and set the asking price to an insane number, in a few weeks he backs down