That would be impingement, of course. Blanton said he didn't have his best stuff last start. Velo was down ~2 mph on sinker.
With a 5.92 ERA and being winless in four starts, it's not like Blanton was exactly making anyone forget about the Fab Four ahead of him in the rotation. But he really hasn't pitched as poorly as his ERA suggests, and presumably would have been a perfectly fine No. 5 starter going forward.
And maybe that will still happen. At the moment, the job goes to prospect Vance Worley, who's been outstanding this month with Triple-A Lehigh Valley. Worley actually pitched 13 innings for the Phillies last September and looked good. According to John Sickels, Worley's "pitches are generally rated major league average, but he has four of them and he throws strikes. I think he can be a serviceable fourth starter."
So being a serviceable fifth starter should be a breeze.