I seem to remember 4 or 5 days ago when he shut down a very potent Red Sox offense for 5 innings defeating Pedro Martinez in his first start of the year. That is slightly above average. I wink
That said I would take Carter anyway as closer insurance. There are other even better middle relief men out there (Donnelly for instance).
If you think you may need a solid starter at some point keep Santana. It's only my personal impression that Santana will get his shot in the rotation sooner rather than later. If that's what you want to know than I think he will get his shot.