It would be beyond stupid to sit Paul Pierce. Give Igudala some credit. His defense has been good on Pierce, and clearly Pierce's knee hasn't prevented him from making huge defensive stops, like the fast break block in game 1.
Stop overreacting just because Pierce is missing shots. He's missing wide open shots, too. I've had knee troubles before, too, worse than what Pierce is having now, and it didn't drastically affect my ability to make wide open shots, which is some of what Pierce is missing. If your knee is bothering you, then you make the necessary adjustments to ensure your shots have a better shot at going in.