Unless Brady was following the same instructions from the medical team that only allowed him to play Miller for 45 minutes against Rochdale, ie "you can't play Arnold yet, its too big a risk".
I'm well out of touch with these things I have to admit, but I would be questioning the sanity of those who decided that Arnold was fit to sit on the bench but not start if that indeed was what was being advised. What if Mitchell gets a head injury in minute one and Arnold needs to replace him. How is that any different from being fully fit and able to start?
Regards Miller, as manager i would be asking the player what he thought and considering his view. I fail to believe any professional player agreeing with a prognosis that their own hamstring will only last 45 mins max. How can you put a time frame on pulling a hamstring in a game of football. It's not like hitting the wall as a marathon runner or going more than ten rounds as a boxer?
Its been a very long time since I did anything on the sporting field but when I was, I would trust myself more than any physio or sports scientist if I had tweaked a hamstring.
Reading books, doing science and learning stuff works to a degree, but they can complicate things at times