RP-to-SP Inning Estimations

There has been some recent news about A.J. Puk (link) and Seth Lugo (link) joining their team’s rotation. Both were full-time relievers in 2022, so it would be quite a transition. The question I wanted answered was not if they would make the transition, but if they did, what are the most reasonable number of innings they should be expected to throw if they make the transition? After looking into some comps, it was more than I expected.

To find my sample, I didn’t want any tweeners, guys who started 15 games and had 20 relief appearances. Both Lugo and Puk were only relievers last season and could be going to the rotation.

Another problem has been a change in how pitchers are used over the years. For example, back in 2005, Miguel Batista was a reliever in 71 games (74 IP) and the next season threw 206 IP in 33 starts with a 4.58 ERA. Only Sandy Alcantara threw more than 206 IP this past season and he did it with a 2.28 ERA. The most innings thrown by a non-Colorado pitcher with a 4.50 ERA or higher was by Nick Pivetta (4.56 ERA, 179 IP).

With those considerations, I set the following parameters to find the comps:

• Matched seasons from 2012 to 2021.
• In the first season, he needed to have pitched in 25 games and been a reliever in 80% of those games.
• In the second season, they need to have started at least 5 games and be a starter 80% of the time.

With those parameters, 18 pitchers met the criteria. Two pitchers, Tyler Chatwood and Carlos Martinez, were removed because they threw fewer innings as a starter than as a reliever. So here are the 16 pitchers who meet the requirements.

Teams don’t mess around with the transition and add ~80 innings (26 starts) to the average pitcher’s workload. If both can stay healthy, it would push Lugo’s projected workload to 145 IP (65 + 80) and Puk’s innings up to 146 (66 + 80).

I can understand the 80 IP might seem high, so just looking at the three pitchers who made the trio added between 46 to 68 IP but they were limited to 18 to 25 starts. Additionally, most of their stats (besides walk and groundball rate) got worse across the board with the 1.6 drop in fastball velocity being the most jarring.

I have my doubts about Lugo and especially Puk making the transition to starting pitcher. I just figured they wouldn’t throw enough to be relevant. If they successfully make the transition, there is some historical understanding of the innings bump they should be projected to see.