"Sheets' health is clearly an ongoing issue. My question is whether he's a decent buy-low candidate right now. What is he worth?"
Tough question. First, I do think he's a good buy-low candidate. His K-to-walk ratio in the games he's played so far this season is unreal. If he can stay healthy, he'll be one of the top pitchers in the league. But that's a BIG "if" at this point. The recent news is encouraging, but there's no guarantee about any of it, and we still haven't seen any timetable for his return.
You may have trouble getting him as cheap, though, as by now 90 percent of his owners are otherwise bullish on him. I suspect those sort of owners are the only ones who would have grabbed and held him given the injuries last year. I'm one of those guys. At this point, I'd rather keep him and take a gamble that he bounces back big, rather than deal him cheap. So it would take a lot more than he's probably worth right now to get me to deal him.
I am the Master. Don't question the Master. Just do what he says and be proud.