My view:
Let's say they had set his minutes for the first 5 games to maximums of: 4, 8, 12, 16 and 22.
What if this was the first game. And you make an exception. Instead of 4 minutes, you play him 8. Then next game "he was able to play 8 last game so he can move on to 12 easily!" - and what if that second game happens to have a close finish and you put him back in there for 2 minutes to win it. He'll have played 14 minutes.
.edit: forgot to make the point that in an extreme case, he could end up with 48 mins while he should have just played 22.
Garnett seems like the kind of player that doesn't take "you can't go out there right now" for an answer. Strict rules *do* work, and they're there for a reason. No need to divert from it halfway through.
Funniest thing, a week or so ago everyone was saying "Doc needs to learn how to limit the minutes to his starters!" (Mainly PP) and now that he does limit an injured starters minutes, you seem surprised?
Doc.
My kind of coach.