In my Man Utd save I have Gaya and Shaw both fighting for the LWB slot in the team. They are very similar in age and ability (Gaya is however, also able to play on the right and is my 2nd choice in that position), I've managed to keep both happy using rotation for the last 4 seasons even using them to substitute the other during a game when returning to fitness (the returnee is always placed in the first team and plays for about 50 mins until fully fit). It's easy to keep your players happy in this way (as in real life getting selected first is more important than use as a sub), you just need to make sure that all the players are given an equally fair shot at the start of a match. I've had the injured player whine as well it's just one of those things you have to live with as the AI is not perfect.