A C+? Are you grading him soley on the Football end of things, which he really didn't fully get involved with until the last few years.
IMO it really wasn't Murphy who tumbled, it was Ted Thompson. Now if you want to blame Murphy for letting TT drive the car into the ditch, so be it. However, I think TT really had full control of everything football related and Murphy was fully in charge of the business side of the Organization. How much those 2 things mixed, is anybodies guess. Thompson was running the ship when Murphy came in and I doubt Murphy or the executive committee ever really saw the need to step in until recently.
Murphy has been praised for doing an excellent job on the business end of things, including overseeing Titletown District development and Lambeau Field expansion.
So the clock has started on grading Murphy on how he handles both sides of his job, but I think its way too early to grade him fully, especially on the Football Operations side of it.