The defense has played well in stretches, but Krstic is a major liability in the middle. He's been terrible -- I mean, legitimately horrendous -- at rotating / switching and cutting off penetration. He looks lost regarding when to help, etc., and he's too slow to recover or to slide over to pick up opponents driving to the hoop.
If Shaq comes back, that hopefully won't be a huge issue. Shaq has some of the same issues (but to a lesser extent), but he's a big, imposing figure in the lane who makes opponents pay with hard fouls. Krstic just plays soft.
However, if Shaq can't come back, here's why Krstic's play is important: our defense is built around our big men, from the inside out. The tough play of KG + Perk or Shaq has allowed our perimeter defenders to play tighter defense on the outside, without having to worry as much that their defensive assignment will blow by them. The team has been successful at sealing off penetration, in other words, which further allows Rondo to gamble more, etc. If one of our bigs -- i.e., Krstic, but really any of them -- starts playing matador defense, that not only allows the opposing center to go off (like Ronny Turiaf), but also leads to the other team running layup drills against us (like Carmelo did several times).
So, let's get Shaq back in there, and if JO can contribute as well, fantastic. Without either (or both) of them, I worry that we're in trouble.