Recently on this site, an article was written by Alex Kienholz about Scott Diamond, and what should be expected of him in 2013. In his discussion, he suggested that Diamond's ceiling is that of a #3 starter. But what does it mean to be a "#3 starter"? Baseball analysts and insiders often use the relative rotation position to indicate the quality of a pitcher, though it has always bothered me that there is not formal definition for such descriptions, as one person's #3 starter could easily be described by another person as a #4 starter, even though both people essentially agree on the underlying talent of the pitcher in question. I have come up with two possible definitions: one rather subjective and the other more analytical. At the end is a poll, and I'm interested what everyone else means when they use the term "#3 starter."
Making the Playoffs
The first definition is basically the answer to a question: Could a team make the playoffs with said pitcher as their Nth best starter? Obviously, this is a rather subjective question, as it depends on a lot of assumptions about the quality of the pitchers ahead of him, the offense and defense for the team, and the strength of the bullpen. However, it does provide a good framework for determining the relative talent of a pitcher. In Diamond's case, it comes down to the following questions:
Q. Could a team make the playoffs with Diamond as their #2 starter?
A. Probably not. The rest of the team (#1 and #2 starters, offense and bullpen) would have to be stacked.
Q. How about as their #3 starter?
A. Probably. If we were assured that Diamond could replicate his 2012 performance, then yes. 2012 Diamond would definitely have been the 3rd best starter for most of AL teams that were in contention last year. However, as Alex noted in his article, there are a lot of warning signs about his ability to maintain that performance. If he pitches fewer innings with a worse ERA next year, then I am much less confident. Hence the wishy-washy answer.
Q. As their #4 starter?
A. Absolutely. Even if he regresses a little bit, he would still be among the best 4th starters in the league.
Based on those answers, it seems reasonably to say that Diamond is a #3 starter assuming he can maintain his 2012 performance, but is at least a very solid #4 starter.
One assumption that can be made when trying to define a #3 starter is to assume that there are 30 #1 starters (one for each team), 30 #2 starters, etc. Using this assumption give a baseline for calculating average performance for each spot in a rotation. To do so, I took all the starting pitcher season stats from the last four seasons who pitched at least 100 innings in a season. I sorted these pitchers by Wins Above Replacement (Fangraphs' version), and picked out the top 480. I broke these into quarters, and then determined the average performance for each quarter. These averages more or less matched my expectations for each position in a rotation:
Just looking Innings Pitched, FIP and WAR, Diamond in 2012 definitely fit in right between an average #2 and #3 starter stats, so I would characterize him as a high-end #3 starter last year. As mentioned, there are question marks about his ability to match that performance this season, but he has a significant cushion before reaching #4 starter levels.
Based on both of these methods, Scott Diamond in 2012 rated fairly solidly as a #3 starter. I would say that his ceiling is that of a high-end #3 starter. If he is able to maintain his performance going forward, he should be able to maintain solid middle-of-the-rotation value for years to come. As was the case with his low-walk, low-strikeout fore-bearers on the Twin's staff (Radke, Silva, Pavano, Slowey, Blackburn, etc), Diamond's future value with the team will depend on his ability to maintain his effectiveness while staying healthy. If he can pitch 175+ innings per year with an ERA/FIP under 4.00, he will continue to be a valuable member of the Twins rotation.
I'm curious which definition for #N starter you preferred, or if there are alternative methods you use when rating starting pitchers.