Shaq is a better post defender, but Krstic is a better overall defender.
Actually I'm surprised with Krstic's defensive numbers, especially on post-ups. Truly stellar. He has less than 100 plays in the database but he ranks top-10 league wide. I haven't seen the film yet, but now I'm curious. Probably a mix of randomness, over-help, favourable matchups and maybe more defensive talent to defend the post than I thought.
Where are those stats from?
A stats service called Synergy.
Looking at a full season of plays (last season), Krstic ranked 19th in the league amongst centers (including pfs who played the position) for post-up plays (minimum 150 plays). That's a more accurate measure of his talent than his rank this season. In this type of plays he allowed 0.87 points per play (with a 44.4FG% allowed, 9.5% shooting fouls committed and 8.6% turnovers forced). Exact same efficiency of guys like Noah, Gasol and Dalembert. For good measure, Perkins ranked 5th with 0.75 ppp allowed and Jermaine O'Neal 7th with 0.75 ppg. This season his numbers improved, but I don't think he did.
So yeah, watching him I think he's an above average but not excellent post defender. The type of defender that will contest shots by raising both his arms and staying on the ground, has enough footwork to not be thrown away of the spot in most situations and isn't easy to back down. He isn't very physical, doesn't bump people before the ball arrives, something that the Celtics do very well when defending Howard, for example. And doesn't really contest the shots, it's more like he bothers. That's generally enough to make one competent, even if excellent gets out of touch.
Anyway, it's the pick'n'roll defense and the help defense at the rim that are more shaky.