Interesting topic, but I don't think you sit Davis for Harangody ever. For starters, Davis does get down on himself if he's playing poorly, and so its key not to show him that you've lost confidence him. Instead, keep the routine and keep getting him minutes.. its the only way he can reliably work out of the funk he's in. We need Davis off the bench in the playoffs more than we need Harangody, so lets get Davis as good as we can.
However, I have been preaching 2 things for quite some time now. 1) Davis should not be getting 20+minutes if everyone is healthy (which they aren't). Ideally I think he should be around 12 minutes a game if we were at full depth. 2) Davis' game is even more predicated on the play of those around him than anyone else. He's a good complimentary player, but would not look good at all as soon as he's surrounded by average/good talent, as opposed to great talent (like we had with Rondo & Kg in there too). This is the primary reason I don't think he should ever be a starting PF for the Celtics, but fortunately its the reason I think we can re-sign him rather cheap (around $4mil/yr) this coming off-season.
So in the end, no, don't play Harangody over Davis.