Simply put, Doc's original game plan of having the offense flow through Ray and Pierce in any manner(outside or inside) was beaten by Pop's game plan of switching Hill to Allen and playing way off Rondo with Manu.
Rondo wasn't facilitating because he was getting the ball to Allen and Pierce. Allen was taken completely out of the game by Hill and Doc never adjusted properly by setting picks on Manu to set up switches or Rondo drives.
The SA defense was very good and the Celtics outside shooting was pathetic, probably in part due to the SA D. Doc didn't adjust his game plan coming out of the the half and got killed. The team also laid an egg but this game was lost before the game started because Pop out game planned Doc and Doc never changed the plan.