As far as I know, Corbin's only had TJ once. As I've said before, the second TJ is really bad news on a long-term basis, especially for starters. Maybe the surgeons are getting better, maybe the rehab's getting better. Maybe not.
One TJ makes the signing maybe better because it fits with their understanding of the recovery process. If they trust the surgeon, saw the x-rays/ MRIs, whatever, they are probably going back to their belief that, on average, TJ lasts about 7 years or so, or at least they used. They probably are figuring that he's got a decent shot at avoiding further surgery. The loss of velo should be concerning, as well as his slider-heavy approach in terms of arm-stress, but they seem relatively smart about TJ.