The Mariners rotation is less stressed than their innings total indicates

The Mariners acquired a grand total of Wade LeBlanc this offseason to augment their glass-boned 2017 starting rotation. Through a combination of excellent internal improvements and relative health from James Paxton and Félix Hernández, the Mariners’ 2018 rotation has been much improved. So improved, in fact, that the concern pendulum has swung towards whether their starters have thrown TOO much and may require a spell down the crucial final stretch. Thanks to well-managed pitch counts and a plethora of low-leverage innings, Seattle’s starter stress situation is probably less serious than it seems.

Following six innings from Marco Gonzales Sunday afternoon, the Mariners crept up from 5th to 4th in all of MLB in innings pitched by starters. That number is even better on a rate basis, as Seattle trails just Houston and Cleveland (albeit by a wide margin) in IP/Game Started. The quintet of Paxton, Hernández, Gonzales, Mike Leake, and Wade LeBlanc have combined for 99/105 starts this year, and 96.1% of the innings for M’s starters.

Seattle’s rotation has been more effective than expected thanks to their unproven triad of southpaws. Their excellence has boosted Seattle into the pennant race, but their lack of track record means it might be irresponsible long-term, and even detrimental short-term, to allow Gonzales, Paxton, and LeBlanc to push towards career-highs in innings-pitched. Scott Servais and Jerry Dipoto have mentioned the possibility of a 6-man rotation. With Erasmo Ramirez looking healthy in his Tacoma rehab, such a move wouldn’t necessitate a trade, but it’s uncertain whether drastic adjustments to the workload are truly needed. For as many innings as Seattle’s starters have thrown, they’ve been strained far less than the average SP on pace for 160-180 IP.

Despite ranking the aforementioned 4th in IP, Seattle’s SPs are just 17th in total pitches thrown by starters. That’s no insignificant separation; it’s one of the most drastic discrepancies for any team. Below, I sorted every team in order of IP by starters (green line), then added the team’s total pitches by starting pitchers (blue line).

Baseball Savant / Fangraphs

So while Seattle’s starters are gobbling up innings like the league’s elite, they’ve been economical with their pitches, checking in at 17th in total pitches per start. Speculating on arm health is all we can do, but at minimum, there’s reason to believe the M’s rotation isn’t approaching an empty tank, despite the “CHECK INNINGS” light flashing ominously. But there’s even more in their favor.

The Win Probability charts we post in our Game Charts after every game indicate the ebb and flow of the game, and help us highlight key moments as high leverage. This is measured on what is commonly called the Leverage Index (LI). Whether you’re a sabermetrics neophyte or diehard, the core concept here is a unifier: certain situations in every game are more significant opportunities to affect the outcome than others, and should be evaluated as such. To pull directly from the Fangraphs explainer on LI, it’s something fans of all ages can largely intuit:

Leverage Index is a measure of how “on the line” the game is at that particular moment. The great thing about LI is that it’s extremely intuitive, even if the calculation might not be. You know when the game arrives at a high leverage moment. If we polled people from various baseball backgrounds (statisticians, players, coaches, fans) they would all generally agree on which moments were high leverage. LI is simply a quantification of that intensity based on Win Expectancy.

This allows you to determine how players perform in different situations (high, medium, and low leverage). It allows you to review the way managers use their relief pitchers. It warns you that the game might change very dramatically. An LI of 1 is average. Anything above 1 is above average and anything below it is below average.

The Leverage Index is often a tool for measuring relief pitchers, as their outings are typically built around massive potential swings in game outcomes. The entire purpose of the expansion of bullpens, in a sense, has been to have a number of pitchers on hand to give maximum effort in those high leverage situations. Starting pitchers aren’t afforded that luxury.

By spreading their production out consistently over six-to-seven innings, SPs typically hold back from full effort unless absolutely necessary. If a starter has two runners on and nobody out they’ll be forced to reach back for a bit extra to maximize their shot at escaping unscathed. The more often a starter has to go to maximum effort, however, the more rapidly they are likely to fatigue. In the micro this can mean an early exit from a game, but the additional stress adds up over a season/career - glances grimly at Félix’s 2009-2014.

As it pertains to the Mariners’ rotation this year, they’ve simply kept things remarkably chill. All five of Seattle’s main starters are averaging Leverage Index scores of under 1.00, ranging from James Paxton’s 0.99 to Félix’s 0.91. In other words, the Mariners’ starters aren’t as likely being stressed unduly by the situations of the game. It’s undeniably an imprecise measurement of stress, but overall Seattle sits at a leisurely 20th in starting pitcher LI as a rotation. As you might imagine, that stress has been transferred onto the bullpen, which has handled the highest leverage situations on average in all of baseball.

Despite eating more innings than any rotation besides the Astros and Cleveland, Seattle’s starters have thrown fewer pitchers than the average starting five. Additionally, those pitches have been, compared to the average pitcher (and average starting pitcher) lower-leverage and, theoretically, lower stress. Seattle can’t afford to rest their horses. Thanks to Seattle’s rotation sticking with the C the Z mantra, they’ll hardly need to.