How Many Starters Do Teams Use in a Season?

One of the bigger stories of the 2023 season to this point, rather unfortunately, involves underperformance; specifically, the story of the highly paid and underperforming Mets. Through July 21st, the $445 million dollar (after tax penalties) Mets own a 34-40 record, and sit 14 games behind the division-leading Braves. While there is time for the Mets to turn things around, that possibility hasn’t staunched the steady stream of stories by the media, or responses by ownership.

One obvious location that frustrated fans and prognosticating pundits (why have a blog if you’re not going to lean into alliteration?) might point their fingers when assigning blame or seeking answers is the Mets rotation. Mets starters own a 5.01 ERA (4.95 FIP) to this point in the season and will simply need to be better should the Mets hope to heat up.

Now, this post is not so much about the Mets, or even their surprisingly shaky rotation, as it is about starting pitching in general. It likely comes as no great surprise to baseball fans that both starting pitcher quality and health are crucial. 162 games is a lot of games! Staying healthy and productive for such a marathon is oftentimes as much about skill as it is about good fortune. For the Mets, among other things, the injury bug bit early and the suspension bug reared its head as well. No matter how strong an Opening Day rotation may be on paper, it takes standout performances and good health to support a winning team.

And that fact begs the question: how much quality starting pitching is enough? Obviously, more is better. But when even Steve Cohen cannot seem to find enough, one wonders how much other teams in the recent past have relied on.

This finally takes us to the namesake of this blog post: how many starters do teams use in a season? In recent years, that number has varied widely.

Since 2015, the number of starters teams most often used in a full length season has been 11. That number seems to be all at once pretty large and yet still unsurprising. What’s maybe more surprising is the long righthand tail of the distribution above: a lot of teams field 12+ starting pitchers each season. On average, teams have used close to 12 (11.8) starters in the past 7 full seasons. The 2019 Blue Jays used the most distinct starters since 2015 with 21.

In the last handful of years, teams have dabbled in leveraging openers, six-man rotations, minor league options, bullpen games and more; all the while they continue to deal with the inherent obstacles of pitcher health, stamina, and effectiveness. Still, on the whole, it bodes well for those teams that manage to keep their number of distinct starters on the lower end, as that is something of a proxy for health and performance.

To illustrate this, the same histogram below distinguishes playoff from non-playoff teams by their unique starter count.

It’s not causation, but having to rely on fewer starters (due to health and/or effectiveness) corresponds to better outcomes for teams. Since 2015, playoff teams have used an average of 11 distinct starters over the course of their season. During that same stretch, teams that don’t make the playoffs have, on average, handed the ball to 12.5 individuals to start games.

Clearly, depth is crucial. While rotations these days are most often of the five-man variety, in actuality many more players take a turn. Next, I looked at how much mileage teams generally do get out of their top 5, as ranked by innings pitched.

In 1,428+ inning regular seasons (assuming all 9 inning games), 1-5 starters (again, as ranked by IP) are pitching less than half that number in plenty of cases. This reality once again drives home the fact that pitching depth is not so much a luxury as a necessity.

Just as there are positive outcomes in getting more starts from fewer pitchers, so too is there value in having a top 5 that covers more innings.

Teams that did not make the playoffs here got, on average, close to 680 innings pitched from their top 5 (IP) starting pitchers; teams that did make the playoffs got ~70 more innings from their top five – just under 750 innings pitched. 

While these numbers are more directional than anything, they indicate the incredible value of acquiring quality starting pitching, keeping those individuals healthy at all costs, and finally having yet more quality pitching waiting in the wings for when plans A through E go inevitably awry.Through less than half of 2023, the Mets have already tapped 9 individuals to start games for them, an ominous sign. But if Justin Verlander and Max Scherzer in particular can improve on their performances and luck to this point, they might be able to provide the steadying force that rights the ship.