Among the many side effects of MLB’s full-throated embrace of sabermetrics has been the rise of the bullpen in October. The old way of thinking — riding your headline starters until their arms could take no more — gradually gave way to the new prevailing tactic — shielding your starter from going through the opposing lineup too many times, and thus dipping into your bullpen early and often.1
But has the bullpen fallen out of favor this postseason? While the days of starters being expected to go the distance during every trip to the hill are likely over, there’s evidence that relievers have taken a step back in 2022. After pitching a whopping 55.1 percent (!) of all innings in the 2021 playoffs, relievers have thrown “just” 45.0 percent of innings during these playoffs.2 And that comes on the heels of a 2022 regular season that saw relievers throw 41.3 percent of available innings, down from the all-time high of 44.5 percent in the pandemic-shortened 2020 season:
With the usual caveats about small sample sizes in the postseason, it could be that MLB teams have corrected some sort of market inefficiency. While relievers are throwing fewer innings this postseason, they’ve also been more effective than ever — and their rotation counterparts have bounced back, too. So far in these playoffs, relief pitchers have struck out more batters and allowed fewer walks and hits per nine innings than in any postseason since 2000, and have allowed fewer home runs per nine innings than in all but two postseasons in that same period. Meanwhile, after seeing decade-highs in categories such as ERA and WHIP in 2021, starters have found a way to stymie more opposing hitters as well.
After decades of October baseball trending toward bullpens, what’s the reason for this relatively sudden reversal? One theory is simply that the teams making deep runs this postseason simply have better starting arms than relief counterparts. But the evidence for this seems mixed. The ALCS-favorite Houston Astros, who had the best starting rotation by FanGraphs’ Wins Above Replacement during the regular season, have actually gotten a higher share of their innings from relievers (56.5 percent) than from starters in the playoffs.3 And while the Philadelphia Phillies, who boasted the second-best starting pitching during the regular season, have gotten a whopping 60 percent of their innings from starters, you also had the Seattle Mariners, who featured the worst starting pitching of any playoff team (again, by FanGraphs’ WAR) during the regular season and yet had their starters carry the bulk of the load during the postseason.
It also remains to be seen whether the newfound shift in usage from relievers back to starters will continue into the later rounds of the postseason — or into future seasons. Under the new postseason format, the league-championship round of the playoffs will feature no off days beginning with Game 3 of each series. So teams may be more reluctant to ride their starters for long innings if they expect to call on them in subsequent games — either as a starter again on limited rest, or from the pen.
At the same time, it’s possible that “bullpenning,” openers and other relief-heavy strategies have reached their carrying capacity in modern baseball. The sabermetric revolution that brought about a new way of deploying late-inning flamethrowers may have at least temporarily leveled off. In the meantime, we have more workhorse performances to admire — such as the eight-strikeout, seven-shutout-inning gem that Philadelphia’s Zack Wheeler slung to start the NLCS Tuesday night — and (slightly) fewer specialists to put the nail in the coffin of opposing lineups.
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