Starting Pitcher Article: Pitchers Underperforming Their SIERA

Earlier in the week, we examined several pitchers who have been pitching better than their SIERA suggests. We’re following that up with pitchers who have been struggling and underperforming their SIERA this season.

The visual below shows the pitchers who ranked in the top 25 among pitchers who have underperformed their SIERA.

Sometimes, it’s luck and skill that could help starting pitchers perform better moving forward. That said, we’ll examine several starting pitchers who have been struggling, yet SIERA indicates better outcomes coming soon. Should we buy low on these starting pitchers?

Jack Perkins, SP, Athletics

In June, the Athletics began using Jack Perkins as a starting pitcher. Since then, Perkins has an 18.4% K-BB%, 35.5% ball rate, and a 11.5% swinging-strike rate. That coincides with a 101 Stuff+ and 101 Location+. Unsurprisingly, Perkins’s Location+ (104) and Stuff+ (102) throughout the season were slightly higher, since a pitcher’s stuff tends to play up as a reliever. 

As a converted reliever, he has only reached five innings three times, but there’s a chance his workload will ramp up as the season progresses. This is a case where Perkins has been unlucky with a brutal 58.6% strand rate, and the projections expect his .331 BABIP to regress favorably. Most of Perkins’s ERA estimators expect an ERA under 4.00, suggesting he has more skills than the outcomes suggest.

As a starting pitcher in June, Perkins lowered his four-seamer (33.2%), sweeper (31.4%), and changeup (13.7%) usage. The sweeper and changeup usage shifted by 2-4 percentage points lower than in May. Perkins also started throwing his cutter 12.3% of the time, seven points more than in prior months. He also added a traditional gyro-like slider, throwing that pitch 7% of the time. 

Pitch mixes can fluctuate throughout the season, so we probably don’t need to examine them deeply. However, we’ll highlight Perkins’s new slider. He has been locating his slider low and away to right-handed hitters and toward the outside edge to lefties in June. That gives Perkins a variation of his sweeper, but the slider drops three inches more and doesn’t move as much toward his glove side. 

Perkins can command the slider well with a 103 Location+, aligning with the heatmaps. The sweepers and sliders crush the stuff models, earning a 113 Stuff+. Besides his sweepers and sliders, the changeup generates plenty of whiffs, with a 21.5% swinging-strike rate. He throws the changeup 24.1% of the time. He buries the changeup low in the zone and toward the lower outside corner for lefties.

That gives Perkins multiple pitches to approach both sides of the plate. It’s hard not to like the skills, stuff, and command for Perkins. The only issues for Perkins are his team context – primarily home ballpark and defense (The A’s rank in the bottom five in Defensive Runs Saved and Outs Above Average). Perkins deserves more attention and intrigue.

Aaron Nola, SP, Phillies

Earlier in June, I wrote about Aaron Nola as a starting pitcher whose stuff and command had improved. While Nola has maintained decent skills (15.4% K-BB%), he is allowing too many balls in play, allowing runners to score, and giving up home runs. Though regression should hit, at some point, it’s not worth guessing “when” with a pitcher. However, Nola’s lengthy track record will continue to have name value.

As a team, the Phillies’ pitchers have been allowing the highest BABIP in MLB (.337) while stranding runners 65.6% of the time (No. 22). That’s similar to Nola’s BABIP and strand numbers. Home runs continue to be an issue for Nola, with a 17.9% HR/FB rate, up over three points from his career average. That coincides with Nola’s career-low groundball rate (39.8%). 

Nola’s 101 Location+ is lower than his career Location+ (104). Specifically, Nola’s four-seam (95) and sinker (95) Location+ have been worse this season. He is throwing his four-seamer lower in the zone by over 1.6 inches, putting it more in the heart of the plate. Nola’s fastball command falling aligns with more home runs and hard contact. That’s evident in Nola’s four-seamer, allowing a 22.2% HR/FB and a .518 wOBA (.422 xwOBA) this season, significantly worse than the career HR/FB (14.6%) and wOBA allowed in 2025 (.345 wOBA, .335 xwOBA) and in 2024 (.266 wOBA, .283 xwOBA). 

Similar to the four-seamer, Nola is throwing the sinker nearly two inches lower in the zone (1.9 inches) this season. Specifically, he is locating his sinker low and inside to right-handed hitters, leading to more balls in the air (17-degree launch angle) and hard contact (.447 wOBA, .360 xwOBA). For context, Nola’s sinker allowed a zero-degree launch angle and a .344 wOBA (.270 xwOBA) last season against right-handed hitters. 

Nola’s knuckle curve still leads his arsenal, with a 124 Stuff+, 108 Location+, and a 19.5% swinging-strike rate. It still generates weak contact against righties (.249 wOBA, .227 xwOBA) and lefties (.288 wOBA, .255 xwOBA). Thankfully, that’s been a consistent part of Nola’s profile, but he needs the fastballs, cutter, or changeup to step up to have more confidence in a bounce-back second half of the season.

Speaking of the changeup, Nola commands it well, with a 109 Location+. However, the stuff models have the changeup as Nola’s worst pitch, with a 65 Stuff+. Since Nola commands the changeup well, it generates weak contact (.193 wOBA), though the .304 xwOBA suggests regression. It’s something different from his fastballs and knuckle curve that he dots low and away from left-handed hitters.

Since May, we’ve seen better stuff (107 Stuff+) and command (106 Location+) with a 15.2% K-BB%. That’s positive to see better stuff and command since May, but the results (6.04 ERA, 1.45 WHIP) have been rough. The 4.13 xFIP and 4.03 SIERA since May suggest better outcomes should occur, but good luck on guessing when. 

I probably wouldn’t be buying low in redraft leagues (though I really want to), unless you’re scooping him up as a bench stash in 15-teamers. 

Jake Irvin, SP, Nationals

I’m living under a rock because I didn’t realize Jake Irvin has a career-best K-BB% (15.8%). That’s supported by Irvin’s career-high swinging-strike rate (10.4%), up two points from his career average. Irvin’s .304 BABIP is increasing with the strand rate (63.4%) falling, negatively impacting his ratios. If Irvin can maintain his K-BB% and swinging-strike rate, he’ll be closer to average than significantly below average from a skills standpoint.

The new pitch for Irvin this season is the sweeping slider, which he throws 16.1% of the time to right-handed hitters. He ditched the traditional slider for a slower sweeper that moves over five inches toward his glove side compared to the slider. The slider has been decent against righties, allowing a .286 wOBA (.245 xwOBA). Irvin’s sweeper is the best from the Stuff+ (113) standpoint as one of two pitches above 100, including his curveball (103 Stuff+). 

Irvin’s sweeper gives him a fourth pitch to throw over 10% of the time against righties. That coincides with Irvin lowering his four-seamer (19.3%) and curveball (18.3%) usage by 5-7 percentage points this season. Irvin also threw his sinker 40.9%, up four points from last season against righties. Though Irvin relies heavily on the sinker to righties, it’s not an effective pitch, allowing a .357 wOBA (.364 xwOBA). Besides the sweeper, Irvin’s curveball is the other pitch that’s effective against righties, allowing a .250 wOBA (.216 xwOBA) and eliciting a 26.9% swinging-strike rate. 

Irvin has been toying with his pitch mix against lefties, throwing more cutters (14.3%) and fewer four-seamers (32.6%). For context, Irvin threw his four-seamer 38% of the time with cutters 3% to left-handed hitters last season. Last season, none of Irvin’s pitches had a wOBA under .350 that he threw 10% or more against lefties. This season, Irvin’s four-seamer (.267 wOBA, .338 xwOBA) and curveball (.153 wOBA, .237 xwOBA) have been two quality pitches to lefties in his arsenal.

He might want to ditch the cutter, though he increased the usage against lefties. Irvin’s cutter has been crushed, with a .410 wOBA (.430 xwOBA) this season after a .503 wOBA (.384 xwOBA) last year in a smaller sample of pitches. The cutter has a 114 Location+, contributing to his average or slightly above-average command overall. Irvin’s cutter tends to be located low and inside to lefties, primarily as a pitch that induces contact (86.5%).

Irvin topped out at 5.2 innings once this season, with most of his outings at five or fewer. That suggests Irvin won’t be a candidate for quality starts, though he had nine (27.3%) last season. He looks more like an average pitcher or a streamer, depending on the matchup. It’s worth noting that Irvin’s SIERA under 4.00 is the most optimistic ERA estimator, with a 4.20 xFIP and 5.16 xERA telling us to be cautious. 

Sean Manaea, SP, Mets

Here’s another living under the rock moment, as Sean Manaea hasn’t been on my radar this season. Manaea has been allowed to start over the past four games, leading to a 4.05 ERA, 1.20 WHIP, and 16.1% K-BB%. Since June 13, Manaea has had a 3.77 SIERA, driving his inclusion in this article. It’s hard to compare to last season’s 60.2 innings, but he teased us with a career-best 24% K-BB%, 12.6% swinging-strike rate, and a 110 Stuff+.

Manaea’s 104 Stuff+ remains in the above-average range, though his slider (121 Stuff+) is the only pitch with a Stuff+ grade above 100 this season. Previously, Manaea’s four-seam (100) and sinker (103) had decent Stuff+ grades, though they’re down in 2026. That’s evident by Manaea’s four-seam and sinker with an identical 97 Stuff+ this year.

Thankfully, Manaea commands his four-seam (104) and sinker (102) well from a Location+ standpoint, similar to his career norms. Manaea’s sinker movement profile looks different. The sinker has an above-average.  downward and horizontal movement profile. That’s evident in Manaea’s sinker, dropping 29.7 inches with 18.6 inches of arm-side fade. For context, Manaea’s sinker drops over seven inches more and more than 1.5 inches of arm-side movement compared to the 2024 season.

With Manaea’s sinker dropping significantly more than throughout his career, it generates a 65.4% groundball rate this season, compared to 41.9% in his career. The sinker mainly elicits weak contact (.282 wOBA, .268 xwOBA) and groundballs (-5 degree launch angle) against lefties this season.

With the Mets’ starting rotation hurting, Manaea should have a reliable role, especially considering they’re lacking depth with the struggles from Nolan McLean, Freddy Peralta, and Christian Scott. Manaea deserves some deep-league consideration, with the most notable changes coming from the sinker.