Partially torn UCL supposedly. That's the same injury Matt Harvey had. At first Harvey considered rehabbing it and trying to avoid surgery (something Roy Halladay did successfully as you probably know). Ultimately, of course, he got it operated on. Don't know how bad Santana's is, nor how likely his arm is to blow. But yeah if it's true that's almost certainly a big part of the reason he's unsigned. It's not just the draft pick.