Rees was not in "over his head" so much as he was limited in what he had on the field. Why Lenzy, Keys and Johnson didn't play is a mystery to all of us except for Kelly's insistence they "check off all the boxes" before being allowed to see the field.
And contrary to popular myth, Book does have the arm strength to heave the long ball, but he was very inaccurate most of the time doing so. Add into the fact he doesn't spot open receivers well (my brother was at the game and said receivers were open all game long) and you have what we have.
I have faith that Rees, with his father's pedigree, his time with the Chargers and the fact that he started many games as a QB, knows what he's doing. He's not the best he will be, of course, but I think he will surprise a lot of naysayers here before it's all said and done.