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Welcome back to Rotoballer's series using Statcast to extrapolate, dig into, and commiserate over data to examine pitching performances. The weekly series will be dynamic as we fine-tune our findings and enlighten ourselves on the information and tools at our disposal.

We're rolling over another month of baseball which means it's time for split talk in Week 15. Since the sample size is large enough for streaks to come and go, market timing is of the essence when scouring the waiver wire and pricing trade targets. To hold myself accountable, the results of our earlier dabble into xwOBA splits were mixed. The ceiling for my risers Caleb Smith (injury) and Tanner Roark were apparently much lower than hoped, while the season lines for fallers Rick Porcello and Alex Wood have remained static.

Fortunately, we did have some success in Week 5 discussing wOBA and xwOBA differentials, so this will be our focus. We'll break up the season from Opening Day through May compared to June 1 through today. For reference, the median xwOBA since June 1 is .324. Let's identify which guys have performed better or worse than they deserved.

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Bound to Pop

All stats for qualified starters that threw over 750 pitches through May 31 and 400 pitches between June 1 through July 9.

Nick Pivetta, Philadelphia Phillies (5-7, 4.62 ERA, 1.35 WHIP, 10.64 K/9)

Few pitchers have experienced the volatility that Nick Pivetta has endured, with his ERA dipping as low as 2.49 and rising up to 4.76. He's nearing the peak of that range now and to his benefit, Pivetta has the league's widest wOBA (.410) and xwOBA (.312) differential of .098 since June.

The disparity between expected and actual results explain Pivetta's horrendous June (7.71 ERA). Possibly losing the touch of his best pitch, the slider, Pivetta reduced its usage from 19% to 12% and depended more on an ineffective fastball that has opponents slugging almost a nosebleed .700. When Pivetta has things working, he's throwing his sliders and curveballs with confidence, generating whiff rates of nearly 20%. Pivetta has started July with mixed results. He's still relying on the fastball too often (60% usage, 4% whiff rate) and remains extremely timid about the slider (5% usage, 20% whiff rate).

Pivetta's surprising start to the year was likely unsustainable due to his fortunate wOBA-xwOBA gap of -.046. He now finds himself at the opposite end of the spectrum. Whether it's mental or mechanical, Pivetta should steer away from the heater and reassert his breaking pitches. An agreeable 3.75 FIP supports an improved outlook. It's on the youngster to execute the game plan.

German Marquez, Colorado Rockies (7-8, 4.92 ERA, 1.40 WHIP, 9.09 K/9)

German Marquez held a modestly unlucky .009 differential through May before it ballooned to a .060 delta since June. The June discrepancy is the second-widest and his xwOBA of .290 is 21st-lowest out of 130 qualified pitchers.

Although he experienced a rough June (6.75 ERA), Marquez generated a step-up in swings-and-misses for both his key secondary pitches. His slider's whiff rate jumped from 9% in April to 19% while the curve spiked from 13% to 22%. Marquez's K/9 leapt from 8.54 to 10.53 in that time frame. For the season, his slider's wSL/C (2.14) is 12th-best in baseball. Where Marquez was bitten in June was that too many batted-balls found holes (.280 BAA), if that mark normalizes to his season-long rate of .261 and he maintains the elevated whiffs, better results may follow.

If this article wasn't focused on advanced data, we could simply point to Marquez's road (2.62 ERA) and home (7.93) splits. The classic Coors Field calamity. Sure enough, his latest two starts on the road have been excellent (14 IP, 2 ER, 14 Ks). Marquez's low ownership rates puts him in streamer territory. By observing his solid xwOBA, increased strikeout prowess and immense success on the road, an opportune time to swoop him up could be just around the corner.

Other possible risers: Ross Stripling (LAD, .043 wOBA-xwOBA), Domingo German (NYY, .034), Shane Bieber (CLE, .027)


Due to Drop

Jon Lester, Chicago Cubs (11-2, 2.45 ERA, 1.17 WHIP, 6.86 K/9)

Jon Lester has defied the logic of sabermetrics all season. On pace to setting strikeout and walk numbers subpar from his career norms (2.13 K/BB), Lester is last on our wOBA (.269) and xwOBA (.359) differentials ranking at -.090 since June. Even prior to his stellar run of seven consecutive wins just recently snapped on July 8, Lester enjoyed a beneficial -.048 delta through May.

So when will the wheels fall off? Despite the significant dip in strikeouts, Lester is stranding runners at an extraordinary 83.8%. Aside from Mike Fiers, he's the only pitcher with a K/9 under 7.0 in the top-20 for LOB%. Lester's eighth-best BABIP of .247 also starkly clashes with his career rate of .295. In terms of batted-ball, Lester surrenders a poor 93.4 MPH FB/LD exit velocity and his 33.1% Hard% would be his highest ever. Lester could be transforming into a wily old vet before our eyes, but the data isn't reassuring.

Lester's FIP of 4.28 is nearly two runs higher than his ERA. The two figures have been at parity for most of his career. Then again, there could be some intangible dynamics at play. Perhaps there's something psychologically advantageous about bouncing throws to first against an offense. If that's the case, we should all start questioning the usefulness of advanced stats.

Kenta Maeda, Los Angeles Dodgers (5-5, 3.24 ERA, 1.25 WHIP, 10.93 K/9)

Kenta Maeda lacks the glaring red flags that Lester possesses. His luck metrics like BABIP (.314) and LOB (74.0%) are reasonable. His 2.89 FIP actually suggests he's been better than advertised. However, Maeda's wOBA and xwOBA differentials have flipped from .022 through May to the fourth-most fortuitous -.067 since June.

For the season, Maeda is getting hammered with a 37.1% Hard%. His respectable 91.5 MPH FB/LD exit velocity has jumped to 93.0 MPH since June. Maeda is serving up line drives at an alarming rate on his two best pitches, the slider (41%) and changeup (29%). Meanwhile, he's only allowed a paltry 21 hits in 28 1/3 innings over his last five starts. On more traditional gauges, his 3.46 BB/9 and depressed 0.67 HR/9 are ingredients for trouble.

Maeda has greatly developed his strikeout ability this season, witnessed primarily by the material increases in whiff rates for his changeup and slider to around 30%. If he can sustain that, the perils of hard-hit balls and the inevitable uptick in HR/FB could be muted. Maeda looks to possess the components of a breakout season, but his June numbers suggest he's trending in the wrong direction. Expect Maeda to remain a serviceable starter season-long, but don't be surprised if he encounters some misfortunes along the way.

Other possible fallers: Chase Anderson (MIL, -.078 wOBA-xwOBA), Sean Manaea (OAK, -.058), Mike Foltynewicz (ATL, -.057)


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