I mean, clearly it's cost the team almost nothing. They have the best offense in basketball. This is all nitpicking...I would love to see an analysis of Mazzulla's "slow to react" issue and what its actually cost the team. I sense you are arguing that its not cost them yet but will going forward.
This is the problem with the Mazzulla criticism - its an argument that the Celtics are winning inefficiently some times and its hurting their margin of victory - its not that he is actually costing the team wins - unless you think they should have an 80% or 90% winning percentage.
but one could argue this either way: the optimist would say the Celtics have blown the doors off of most teams; this proves their offense is great and needs little to no adjustments. The pessimist would say their weakness is against high level teams and coaches especially in close games down the stretch and they've had almost no experience in real game situations with that this year AND this weakness will be exploited at some point in the playoffs, likely the finals (if they get there).
Maybe Joe is out-thinking everyone and is saving his adjustments for the playoffs, but teams that can guard the 5 out like Denver, the Clippers, and OKC worry me a bit, especially in a 7 game series where adjustments are likely going to be necessary.