Bryan Woo Has Lefty Issues... Again? The Case For David Peterson

Bryan Woo, Zach Eflin, David Peterson

• Mariners Bryan Woo is human. 5.56 FIP over his last 4 starts compared to a 2.85 FIP in his previous 10. To righties, his sweeper usage is down to 3%, a big drop from 16% in that larger 10-outing sample. This was his go-to two-strike pitch to righties. Instead, he’s throwing his short slider 25%+ in those two-strike spots, and it’s been essentially untouchable compared to the sweeper (.080 xwOBA vs a .400 xwOBA for the sweeper). Woo’s larger issue comes against left-handed hitters, where his K-BB has fallen from 25% to 8% and his whiffs have tapered off. He’s also throwing more of his short slider to lefties and reducing changeup usage. 🔱

  • Woo’s short slider is used early count as a strike-stealer away from lefties and in late-count spots as a whiff pitch down in. His heatmaps show the pitch hasn’t been thrown as much down-in to lefties in his last 4 outings. Given his fastball has also moved more toward the outer third of the plate (see below), I wonder if there’s something mechanically off causing his glove-side locations to veer away from his intention (more center to inside versus lefties). I’m ultimately not too worried here, most pitchers will go through stretches like this, and Woo’s problem is pretty focused on lefties. I am curious why he’s turfing the sweeper, however. ~3.50 ERA rest of season feels appropriate.

Bryan Woo’s four-seam location to left-handed hitters, comparing the last 4 starts to his first 10 of the season.

• Orioles Zach Eflin has been great over his last 3 starts (19.2 IP, 4 ER, 2 BB, 18 K). He’s pushing his changeup against lefties in these last three outings. Usage went from 12% last year to 19% earlier this year to 31% recently. He’s using it as his main behind-in-count pitch, which is a clever way to keep hitters off his fastballs when they have the advantage. Against righties, Eflin has pushed his sweeper up from 20% to 37% in these last 3 outings. He’s also backed off his curveball, which was consistently a 10-15% pitch last season and earlier this year. Similar to the behind-in-count changeup to lefties, he’s also throwing his sweeper more in hitter-advantage spots to righties. 🧐

  • Most of what Eflin is doing here should probably be taken with a grain of salt because it’s in a small sample, but I like the execution of fewer fastballs in hitter-advantage counts, especially since he’s maintained his strike rate. I think there’s minimal upside here because of his strikeout rate (he’ll be lucky to crack 20%), but I’m fine projecting better than league average performance around a 4 ERA rest of season.

Zach Eflin’s split-change grip (2025)

• Mets David Peterson now has a 2.49 ERA in 79.2 innings after a CG shoutout of the Nationals. Peterson confuses me because his stuff grades poorly, he has an average Location+ score, and yet we now have a 200-inning MLB sample with a sub-3 ERA and peripherals suggesting he’s more like a ~4 ERA arm. On the surface, his extension (7’+) might help in some way that Stuff+ isn’t capturing. He also throws 5 pitches >10% in every count-state (“pitchability” if you will). 🤷‍♂️

  • Peterson seems to have an ability to not miss into damage zones. I theorize his miss distances and direction (something not tracked publicly) are plus. This results in very little damage (6% barrel rate is top 10 lowest in MLB among qualified SPs). The only way to back this up with public data is to see that he’s in the shadow of the zone more than 92% of pitchers, near other how-are-they-doing-this lefties like Patrick Corbin, Eduardo Rodriguez, and Jeffrey Springs. I’d guess this is why his Location+ is low (it values high zone rates), and yet the results are fantastic. As I write this, I think I’ve convinced myself that he’ll continue to overperform his ~4 ERA projections.