How did the shortened season affect 2021 inning distributions?

Coming into the season, one of the biggest stories that many people, myself included, were on the lookout for was how the return to the normal, pre-pandemic season length might affect pitching staffs. After American League pitching staffs averaged only 519 innings total in the shortened 2020 season, the 2021 season would require, using the 2019 average of 1446, an increase of more than 250 percent. As many expected, injuries that required stints on the injured list were on the rise, at least in the early days of the season — driven in part due to an overabundance of caution, but also in large part due to an increase in major injuries compared to last year.

Cognizant of these early-season discussions and abundantly aware of the amount of injuries that the Yankees pitching staff has faced this year, I decided to dive into the numbers a bit to see if there was a difference in how teams have divided up innings relative to previous seasons. To do this, I found the percentage of innings thrown by the eight pitchers who threw the most innings for each American League team in 2019 and 2021, which included a combination of starting pitchers, bulk pitchers, and high-leverage relievers (I had originally opted to use just starting pitchers for this, but the prevalence of openers in 2019 made this distinction difficult to use).

In the process, I expected to see a drastic decline in percentage of innings thrown across the board, anticipating that a combination of injuries and innings limits in an attempt to prevent injuries would result in teams using more pitchers on a regular basis throughout the season. What I found, however, was something else entirely:

As expected, we see that the pitchers leading a team in innings have seen a dropoff in percentage of innings thrown, albeit not as significant of a dropoff as might have been thought. After these top two — which are, throughout the league, almost exclusively two of the team’s top workhorses — the percentage of innings thrown by the next five actually increased relative to 2019, albeit slightly. This has resulted in teams needing to use the “back” of their pitching staff (everyone after the pitcher with the eighth-most innings) about two percent less than they had in 2021.

This was the exact opposite of what I expected to see, and it also appears to fly in the face of the widespread injuries that we have occurred this year. So what is behind these trends? Let’s zero in on the Yankees and use them a case study, this time expanding our data set from the top eight pitchers to the top twelve.

Note: The top image is from the 2019 season, the bottom from the 2021 season prior to the game on September 6th.

In 2019, the five pitchers who threw the most innings for the Yankees were also the five starting pitchers for most of the season, followed by a pair of middle relievers who often threw multiple innings in Luis Cessa and Chad Green and a long reliever who acted as a bulk guy, Néstor Cortes Jr.. After them, everybody else on the list was a relief pitcher. In 2021, by contrast, only the top four are starting pitchers, with the three other pitchers that made a significant number of starts — Cortes, Corey Kluber, and Michael King — being scattered throughout the rest of the top ten.

Of course, the 2019 season was notable in that almost every time the Yankees needed to tap into their starting pitching depth, they opted to use an opener, which throws these trends off. So let’s dial the clock back to 2017 and 2018:

Note: Top image is from 2017, bottom is from 2018.

Despite the prevalence of the opener potentially throwing innings counts off, both the 2017 and 2018 inning distributions look closer to the 2019 season than 2021: the top five in innings pitched were all starting pitchers, and for the most part, the remainder were used in relief. The exceptions were Sonny Gray in 2017 and both J.A. Happ and Lance Lynn in 2018, all three of whom were acquired at the trade deadline and thus did not have enough time to enter the top five in this category.

So what does all this information tell us? About the Yankees, I can confidently say that a significant chunk of the rotation has stayed healthy for the majority of the season, but that the fifth spot, and to a lesser extent the fourth spot, have seen multiple starters cycle through. I can also say that the elite arms in the bullpen have seen significantly more work in years past (Jonathan Loáisiga and Chad Green have already thrown more innings this year than the top bullpen arms have in years past), a number that is even more significant when you realize that the Yankees bullpen has provided only 41 percent of the team’s innings (down from 46 percent in 2019).

As for the entire league, however? In truth, the change in innings distribution relative to 2019 is small enough that it might just be the result of random chance, the normal year-to-year fluctuation based on how a season collectively plays out across all fifteen teams in the American League. If that’s the case, well, then maybe the jump in innings did not prompt teams to alter their approaches all that much after all.