Obama's in a real tough spot with this, one he enthusiastically made himself:
If he doesn't credit Bush at all, he implicitly takes credit for policies and strategies he previously (and repeatedly) said would not and did not work.
If he does give Bush any credit, it's an admission that if it had been up to him, Iraq would be in a complete shambles with the US watching helplessly from afar.
Either option provides ample fodder for his political opponents to pummel him.