That's true, there are situations where people could only make predictions and assumptions and we could not really know what's going on. This is why I keep telling people that Maguire could be actually better than what we think as well.
Think about it, that dude played under 3-4 different managers at United, and all of them kept him, if he was as bad as we think he was, don't we think that managers would just cut him from the squad the moment they get a chance to do it? They would all tell the team to get a defender urgently and get rid of him. He must be great at training to keep getting a chance, obviously injuries to other players matters in that sense too. We will never be able to understand what a manager thinks when they do something.
It might be hazardous to make big changes to a winning team. To maintain the successful formula while making the required alterations, a balance must be struck. It is difficult to make judgements because, as observers, we do not have a complete understanding of what takes place during training sessions. Garcia choices are clearly a fine line to walk. He becomes a hero if his adjustments are successful, but he is under pressure from both the public and himself if they are unsuccessful. Taking over a team that was dominant the previous season can be difficult because the new manager has more to lose than gain.