We're a bit in the spin cycle, here, as it seems that you credit relatively more of the Spurs' offensive success against the Cavs to Duncan than I do.
It's not that I give more credit to Duncan for helping Manu + Parker ...
It is that I think Duncan could have done that if needed. He could have generated more offensive opportunities for guys like Manu + Parker, or Pierce + Ray, if his team needed him to. Not that he did that in the 2007 Finals but rather that he is capable of it.
Rondo could get into the lane, but he was inconsistent and wasn't really a scoring threat yet.
Rondo had huge problems playing against Cleveland's two long shot blocking threats. Remove one of those players from the equation in the Posey at PF lineup, either because they're on the perimeter or because Cleveland have gone small, and Rondo will be a lot more effective taking it to the rim. Both as a scorer and as a creator for others. Which means more high percentage shots in the paint and on the perimeter on drive and kicks.
Additionally, Wally Szczerbiak would have a lot less help in containing Ray Allen which would have made his life a lot more difficult. Ditto for LeBron and Pierce and how LeBron chose to handle that would decide how much better Pierce played (disciplined man-to-man D or take more team defensive responsibilities).
Cleveland would have a very difficult time handling Tim Duncan in the post against that lineup. They wouldn't be able to send a second large player in a double. They would have been more readily punished by shooters or by dribble penetration (Rondo) if they doubled from the perimeter. Similar to the problems the Cavs had against Orlando in the playoffs last season with Dwight Howard and Rashard Lewis.
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When Boston played big with Duncan and Perk ... I don't think there is much of a difference for the Celtics.
I agree with you that Kevin Garnett would have remained more efficient and very effective. But I also think Duncan would command more of the ball, more double teams, and create more shot attempts for both himself and others. And that the value of those extra shot attempts outweighs KG's individual advantage with his efficiency.
Duncan's lower efficiency + added better shot attempts ... versus ... KG's higher efficiency + lower shot attempts from teammates (who were struggling)
Just think back to how hard the Cavs ran double teams at Garnett whenever he posted up ... now imagine him doing that far more often and the havoc that would create for Cleveland. That is what Duncan will do. So even though you lose some of KG's efficiency + jump shooting threat ... I believe you gain in other areas too which negates that loss.
Overall, I think there is no real difference between a Perk + Garnett combination and a Perk + Duncan combination when playing against the Cavs. A difference in style of play and methods to achieve results, but overall, a comparable end product.
I do think there is a major advantage with Posey at power forward alongside Duncan. I also think Garnett could have had a similar advantage if Boston chose to use it + Garnett committed himself to be a scoring threat in that lineup.
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Anyway, that is my belief of how the matchups would have played out with Duncan in KG's place against Cleveland.