Let's explore this train of thought.
Say Amass does make a mistake, then what? Should we send him back to the U23 squad, never to be seen again this season? Martinez made some mistakes in his first few appearances for United, as did Evra and Vidic. Every young player, whether that young player is 22 or 17, has room to grow as a professional footballer and there's no better way to grow than to play.
If we're going to commit to a promising young player, he has to be given a chance to play and learn from his mistakes. That doesn't make him an automatic starter, but it does mean holding him to the same standard as everyone else. If he makes a mistake, don't fukk with his head by benching him for a long spell. If he makes a number of mistakes, then back to the bench behind whoever it needs to be or back to the U23. No double-standards for anyone is the only way we're going to progress as a squad.