There is no silver lining to a potential Shelby Miller shoulder injury. (The only thing that even kind of counts is that the Cardinals don't seem to be terrified yet.) But if you're looking for a silver lining to redeem if and when he's not interested, it's this: People might remember just how good a starting pitching prospect Trevor Rosenthal is.
It's true: Rosenthal is more obviously a great relief prospect—really just a great reliever. In 31 MLB innings last year (counting the postseason) he struck out 40 batters, walking just nine, for an ERA of 2.01. That'll do, certainly.
But on his way up to the majors, mostly in AA Springfield, he made 20 starts, striking out 104 batters and walking 42, for a 2.97 ERA. I'm willing to stipulate that Rosenthal could be valuable right now as a relief pitcher, probably the second-best one on the roster. And I'm certainly not going to suggest that it will never make sense to view Rosenthal as a reliever—he could yet wash out of a rotation.
But Shelby Miller's not-quite-injury is a reminder that rotation depth is kind of an illusion, and that Rosenthal could be called upon to make starts nearly as quickly as he could be called upon to replace Victor Marte. I'd like the Cardinals to keep that option open as long as they possibly can.