You are bringing up two issues:
Is Hill's health more vulnerable starting or coming off the bench?
Will D'Antoni shock everyone and go to a conventional lineup with a big to go alone with Amare?
IMHO, a player who plays the same minutes is LESS likely to get injured than coming off the bench because he has a chance to get warmed up.
Getting Hill probably ensures that D'Antoni will play the small ball lineup more because he will have the talent to make it work. I would hope he goes big against teams with major low post threats, but that is fewer than a third of the teams.
To me he is more likely to get injured staring and logging 35 minutes a game because of the nature of his injuries. It's not a warm up type injury. Your talking about his ankles which is more wear and tear.
You're able to state this with confidence based on D'Antoni's history of developing his bench and not overplaying his stars, right?
Bottom line is that adding Hill adds more depth. If I see the same crap out of D'Antoni and his bench management next season, I will be joining the fire D'Antoni bandwagon. Then again, if that means Diaw getting less minutes and he continues to play like crap...maybe that's a good thing.