D-backs Game Preview, #18: 4/26 vs. Dodgers

Over the years, it has generally been the case that starter ERA is higher than reliever ERA. There are a few reasons for this. Starters throw less hard, are seen multiple times by hitters, and are less likely to have the platoon advantage. However, the gap had been narrowing of late. As Jack posted Sunday on Twitter, “Over previous 4 seasons the gap between SP and RP ERA had shrunk dramatically. But right now RP ERA is 3.36 and SP ERA is 4.07. The gap has never been that large. Here is a table since 2000.”

The Diamondbacks this year, are not part of this increase. Indeed, the complete opposite is true. Even after yesterday’s game, the Arizona rotation has been considerably better than the bullpen. Here are their stats to date:
Starters: 78.1 IP, 59 H, 27 R, 22 ER, 6 HR, 33 BB, 66 SO, 2.53 ERA
Relievers: 72.2 IP, 72 H, 44 R, 35 ER, 8 HR, 29 BB, 49 SO, 4.33 ERA
That’s a -1.80 difference. It’s not entirely unprecedented, however. As recently as 2019, the D-backs starters had a better ERA; however, the gap was only -0.03. But it also happened in three consecutive seasons, from 2008 through 2010, and in the last of those, the gap was comparable - mostly because the bullpen was terrible. They had a 5.74 ERA, while the rotation was down at 4.40, a gap of -1.34.

There are other teams in a similar situation to the D-backs this year. For example the Mets’ starters have a 2.29 ERA, and their relievers are at 3.45. But those with “reverse role splits” are outnumbered by the ones whose rotation have the normal, higher ERA - all the way up to the Cubs, whose starters are more than three runs worse (5.60 vs. 2.54). The Pirates and Reds (both +2.75) are also driving the overall split higher. If we look at the underlying stats, however, while the rotation has still been doing better, the gap for Arizona is not as large as it seems by pure ERA.
Starters: ERA 2.53; FIP 3.64; xFIP 4.00
Relievers: ERA 4.33; FIP 4.63; xFIP 4.81
FIP is fielding-independent ERA, xFIP is the same, but with an average HR rate. You can see while the ERA gap is -1.80, for FIP it’s just below -1.00, and by xFIP is down to -0.81.

This does suggest regression is more likely to bite the rotation than the bullpen. However, the innings pitched are already “in the bank”, and their results cannot be changed. It’s what happens going forward that will determine the future success of both starters and relievers for the Diamondbacks.