This idea that you “have nothing to prove” needs to be re-evaluated. If this is how you think then you need to re-think your ideas. Every time you step on the pitch you should have something to prove! This applies to all players at all levels of play.
In fact, this is one the areas that separates average players from good players and good players from great players. In addition, it becomes obvious who the players are that become comfortable with the idea that they have nothing to prove. What tends to happen with them is they plateau and you don’t hear about them or notice them anymore. In essence they are no longer “game changers”.
Hall of famer Michael Jordan would bring his A-game even when facing the smaller franchises. For instance, when the Chicago Bulls played the Vancouver Grizzlies (now the Memphis Grizzlies), he would go on a tear and score 40+ points, sometimes half of which came in the fourth quarter. Hence, there was never an off night.
The point being, whether you play for Manchester United or a local club in town there will always be a coach, scout, parent or fan watching. Even more so, you should play for yourself. The idea that you have nothing to prove implies your best days are behind you. Yet there is always something you can do to better yourself and help your team win.