People have said that Duncan's defense isn't that good this year, I think it's not so much about Timmy but the fact that he keeps having to pick up the defensive slack for the other Bigs. You've seen Duncan trying to knock some sense into Bonner when he blows his assignment and leaves Duncan out to dry.
At least Duncan has an excuse when Bonner's on the floor. Duncan hasn't done any better against Amare, Aldridge or Dirk than Bonner has. Duncan got a block last night, so now they're tied with one.