I think the point is that if you want an insurance policy, you want someone better than Pivetta.
But then you'd be starting that pitcher, basically. And if you weren't (meaning they're in the pen) you'd have to stretch them out to start. Unless they were at AAA, but there, you'd call them up if they were effective starters at the ML level.
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Right now Sale, Bello, Paxton appear to be locks for the rotation based on performance so far, dedicated role, and upside.
Filling out the final 2 or 3 spots are 4 pitchers: Houck, Whitlock, Pivetta, Kluber. They're all stretched out to start. Whitlock appears to be headed for the rotation which leaves Houck, Pivetta, and Kluber. Of them, on the basis of stuff/pitching history/routine, I'd guess Kluber does not work out in a bullpen role. (But who knows?)
"Emergency Depth" or insurance policy depth would include Winckowski and Crawford, who are not currently stretched out. It would also include Mata/Murphy, et al. in the minors, who are stretched out as starters but not doing great. It seems there's nothing immediately available in that group.
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Personally I think that trading one of Pivetta/Kluber for a reliever might be something to explore. I don't know if that trade is out there or not. Ideally you'd want to keep the better one. Kluber's at the end of his career (37) has a club option for next year and is being paid $10M. Pivetta is 30, is being paid $5M and has one year of arb left before he's a free agent.