Maybe the Spurs' biggest need this off-season was to get someone to take the pressure off of Tim defensively; someone capable of legitimately guarding the bigger, longer, post-players of the world.
Davis does not fill that role.
Blair should help with the rebounding but hasn't proven to be much of a defensive-player. He's capable of being that immovable object once his feet are set and he's between the his man and the basket, ala Davis, so he can at least force the opponent to make a shot over the top, but that's not exactly comforting. Especially considering the post-players on the current champion are skilled and coordinated enough to not have much of a problem shooting over smaller, stout, defenders.
So, Davis can move his feet pretty well and use his girth in the post fairly well when given the opportunity, but he's not a good defender. Nor is the foul-prone college player Blair, who will also have his rookie status going against him. (And please don't cite how someone plays Tim as proof of being a good defender. Anyone who watches Tim, especially during the regular season, knows how much harder he makes things on himself and how good average defenders can look on him from time-to-time. I guess he likes a challenge?)
So unless you're going to tell me that Mahinmi is going to be able to stand up to the bigger front lines of the world or that Haislip is all of a sudden the Bruce Bowen of 4's, the Spurs have got a pretty big void to fill if they're going to make Holt's money/generosity actually look like a wise investment.
Tim's health is a major concern going forward and the load of his, especially defensively, has to be lightened.
'Sheed was ideal because of what he did on both ends of the court (for Tim defensively and overall spacing offensively) but since he's out of the picture, they've got to at least find a legit big defender for Tim. (Trade or otherwise)
It's a must.