We are a little thin at third. And the baseball gods tend to test teams where they are most vulnerable.
If Johnson got hurt or regressed severely, presumably the first replacement would be Pena (perhaps in a platoon with Pastornicky). Not a great alternative, but not awful either.
Beyond that none of the guys we have playing third in AAA and AA (Salcedo, Kubitza, Leonard) look like they are close to being ready. Nor do we have someone in the upper minors who could plausibly be moved from another position to fill in at third.