It's not, really, players should also take care of themselves a bit. Koeman and his staff got sacked. They fucked up big time, but how does that make Pedri better?
I know in theory the players should trust their managers and physios etc., but like in life, you also need to learn to protect yourself from incompetents or poor professionals making decisions that impact your career to a great degree.
It's not unprofessional for a player to say he has discomfort, or that he needs rest. It's normal for very young players to not play 72 games in their first season. The anomaly is for them to be played so much so sudden.
Thing is, he probably felt pretty OK, and he is so young and probably never had any injuries of that type before. It's impossible for him to know.
It is 100% on the staff and 0% on Pedri
I saw you write in another thread that Koeman is "alpha". I disagree very much, he is a beta who is very eager to "look" alpha, he seems like the type that could take such a request from a young player the very wrong way. So on top of not being natural for a professional player to make such a request, and on top of him probably not even feeling very tired himself, it could very well be a very stupid thing to do. He must trust those who decides.
And the thing is, with such injuries, they aren't really acute, but show themselves a bit later. It is not the case that a player can play 3 times a week for half a year and if he isn't injured then, everything is fine. No, it can lead to more long lasting problems, that show up later. There has been a lot of talk about this, with young players getting too much time too early, doubling up with teams etc.