Reloading from a backup as far as I know would require activation and hence patching again.
One way around it would be if you can install the current patch-version of the game on a second computer and keep that computer in offline mode.
Unfortunately the steam-backup feature is not designed with rolling back patches in mind. It's more for people installing their games on a new computer (not wanting to do the whole download again).
EDIT
Valkrionn seems to suggest this is not the case, so maybe I'm completely mistaken. Whatever the case, playing it safe for a couple days doesn't do much harm, and mitigates the risk.