Seems I didn't express myself too clearly. I don't think he wants him as a scapegoat, he needs him as a buffer.
If Rumsfeld is replaced with anyone else, all the blame for the things that would still happen (or old things that surface) down there would go directly to the Commander in Chief.

And if he should win the election (god forbid!), there's no use in already damaging the reputation of Rumsfelds successor.