Well, it's as simple as this.
We can't make excuses for a team that is still capable of winning, and Jermaine O'neal and Krstic are very suitable for helping to get us a series win over the Knicks, or any team for that matter.
Does Shaq make this team significantly more dangerous? Yes, he does. Does Shaq help us both offensively as well as defensively? Yes, he does. However, he isn't entirely a better defensive Center than Jermaine O'neal. At least, certainly not in the area of shot blocking and pick and roll defense.
That said, we have plenty of strong pieces on our team to beat any team that's currently in the playoffs. We have no excuse. It sucks to not have Shaq, but let's not use it as a crutch.