Can you really overuse your bullpen?

It’s no secret that the Yankees have relied on their bullpen to shoulder a heavy workload this season. Nine different relievers have already thrown at least 35 innings to date — Aroldis Chapman, Adam Ottavino, Zack Britton, Tommy Kahnle, Chad Green, Luis Cessa, Nestor Cortes Jr., Jonathan Holder, and David Hale.

All have performed admirably to date, with the lowest ERA+ (71) belonging to Jonathan Holder, who had a 133 ERA+ last season and who has largely been relegated to low-leverage situations until he gets himself in order, much like Green was earlier this season. However, the question has been raised by many about just how long the Yankees can continue to ride their bullpen before it starts to crack under the workload.

In order to investigate the concept of bullpen overusage, I selected a sample of 50 relievers from the top teams in the American League in the 2017 and 2018 seasons. While not exactly a random sample, I wanted to make sure we were basing this study on relievers who not only were pitching meaningful innings at the end of the year, but who were most likely to be relied on by their managers for high leverage innings.

I then gathered data on their total innings pitched, average number of days rest for the season, K/BB ratio for the season, and K/BB ratio for the month of September (I used K/BB ratio as it is a defense-independent stat, and I used the month of September because most relievers did not have a large enough of a postseason sample size to analyze). After gathering the data, I created two graphs, one graphing the difference between season K/BB and September K/BB as a function of total innings pitched, and the other as a function of average days of rest.

The data reveals...absolutely zero significant correlation between total innings pitched or average amount of rest and performance drop off in the month of September; in fact, roughly as many relievers saw a performance increase at the season’s end as saw a decline. Although some performance variation here may be the result of fatigue over the long haul of the season, this would have to factor in on an individual basis.

Perhaps this information should not be all that surprising. The maxim about relief pitchers is that they are incredibly volatile, and although front offices have gotten better at identifying which relievers are legitimately good and which ones are just the product of luck — just look at the Yankees bullpen over the last five seasons — there does remain some truth to it.

Much like how the month of September and the postseason are largely driven by whichever team gets on a hot streak at the right time, bullpen performance too will fluctuate wildly based on hot streaks by the pitchers. That is not to say that a manager ought not to work on keeping his bullpen fresh — the best performing bullpen is a healthy one, after all — there should not be an overwhelming concern about wearing out a reliever too much before the playoffs.