Count all the games where Harper came to bat late in the game where a hit would have put us ahead, but he struck out. If he had produced a third of the time (not an unreasonable thing to ask) that's at least five games right there.
I think you are completely mindfacting this. Do you have anything to back up your theory that he is hitting worse in those situations. He is OPSing .843 in high leverage situations, which is down a bit from other situations, but no where near what you are making him out to be. Didn't he just homer to tie a game, and then knocked in the walk off run?
Starting too many games with non-MLB pitchers and blowing a few saves because we fired half our bullpen is really where they blame is. A legitimate #5 starter with Helickson spot starting would have given us a good shot at 7 more wins.