I don't get some of the responses in this thread. The OP asserts Tim's defense has fallen off, and people respond with essentially, "You're stupid. Look at his offense." That doesn't address the issue at all. Tim's been horrible guarding the PnR for a couple of years now, but teams didn't really seem to catch on to that until the GS series. Part of Tim's struggles are physical, his lack of mobility affecting his ability to get up on the ball-handler. But part of it is his unwillingness to show hard to contain penetration. He constantly sinks to protect the drive, but his mobility is so bad that guards can snake-dribble right past him. Tim compounds this problem by going for chase-down blocks that he rarely gets, often resulting in a foul.
Duncan needs to just commit to hedging strongly and relying on his help to have his back protecting the rim from the roller. Right now, he's not helping anyone by being in no man's land. This explains the rebounding thing, too, as he is often too far away from the basket to get the board (because despite what some folks think on here, rebounds don't just magically teleport to Duncan's hands just because he's on the floor).