Wei-Yin Chen debuted for Baltimore in 2012, posting a solid rookie season: Across his 192.2 innings, he maintained a 4.02 ERA and a 4.42 FIP. While he did well in many areas, he struggled to prevent runners from scoring, with a 72.8% strand rate and -0.7 LOB-Wins (placing 43rd and 65th, respectively, in baseball). Still, that didn't tarnish his future much — after all, not all pitchers can overperform.
In the two-plus years thereafter, though, Chen's made some changes. Over that span, his 78.2% strand rate (9th in the majors) has translated to 2.1 LOB-Wins, which ranks seventh among pitchers with 300 innings. Notably, that latter mark bests Gonzalez, among others of his ilk. So how did Chen go about attaining it?
Like fellow Oriole starter Chris Tillman, Chen has better limited base runners. In each of Chen's years in the show, he's permitted a lower amount of stolen base attempts:
| Year | Chen SBA% | MLB SBA% | Chen SBA+ |
|---|---|---|---|
| 2012 | 4.9% | 6.5% | 75 |
| 2013 | 3.5% | 5.5% | 63 |
| 2014 | 2.2% | 5.7% | 38 |
| 2015 | 2.7% | 5.8% | 47 |
But this doesn't begin to account for the entirety of the change, because Chen's also posted better results overall when the pressure comes on:
| Year | Bases Empty wOBA | Runners On wOBA |
|---|---|---|
| 2012 | .316 | .317 |
| 2013 | .352 | .301 |
| 2014 | .338 | .290 |
| 2015 | .328 | .266 |
Let's look into this. We'll start with 2012, in which Chen didn't dazzle, and for good reason — he performed worse than league average with runners on, in many regards. He walked more batters...
| State | Chen uBB% | MLB uBB% | uBB%+ |
|---|---|---|---|
| Bases Empty | 6.1% | 7.1% | 86 |
| Runners On | 8.2% | 7.9% | 104 |
...didn't strike out as many batters...
| State | Chen K% | MLB K% | K%+ |
|---|---|---|---|
| Bases Empty | 19.7% | 20.7% | 95 |
| Runners On | 17.6% | 18.8% | 94 |
...and sacrificed more home runs:
| State | Chen HR% | MLB HR% | HR%+ |
|---|---|---|---|
| Bases Empty | 3.3% | 2.7% | 122 |
| Runners On | 3.9% | 2.7% | 144 |
Examining Chen's situational pitch usage gives some clues as to the cause of this:
| State | Fourseam | Sinker | Slider | Curve | Split |
|---|---|---|---|---|---|
| Bases Empty | 59.9% | 4.6% | 15.3% | 7.4% | 12.8% |
| Runners On | 57.6% | 8.7% | 14.7% | 3.7% | 15.3% |
The sinker and splitter replaced, to varying extents, the four-seamer and curveball. In the case of the latter, that decision likely served him well: Chen's curve went for a strike only 55.3% of the time, induced a whiff 8.0% of the time, and left the park 1.6% of the time — the worst rates of any pitch in his arsenal. The second-worst? Those belonged to...his sinker, which possessed a 60.6% strike rate, 8.3% swinging strike rate, and 1.6% homer rate.
Chen's splitter did well enough in terms of whiffs (12.3%), strikes (63.2%), and long balls (0.7%). On the flipside, his four-seamer went for strikes even more often, at 30.2%, while racking up a fair amount of swinging strikes (10.0%) and going for a home run 0.9% of the time. Overall, the switch didn't work out for him — the sinker's awfulness cancelled out that of the curveball, and the splitter couldn't make up for the four-seamer's absence.
Now, we'll compare Chen's 2012 to his 2013 through 2015. In this three-year span, he's flipped the script in all areas — he has doled out fewer free passes with runners on...
| State | Chen uBB% | MLB uBB% | uBB%+ |
|---|---|---|---|
| Bases Empty | 6.0% | 7.0% | 87 |
| Runners On | 4.3% | 7.5% | 57 |
...has punched out more batters...
| State | Chen K% | MLB K% | K%+ |
|---|---|---|---|
| Bases Empty | 17.2% | 20.9% | 82 |
| Runners On | 20.2% | 19.1% | 106 |
...and has done a better job of keeping the ball in the stadium:
| State | Chen HR% | MLB HR% | HR%+ |
|---|---|---|---|
| Bases Empty | 3.4% | 2.5% | 137 |
| Runners On | 2.8% | 2.3% | 122 |
So what has allowed Chen to reverse his fate in this manner? Again, we can turn to his repertoire for the answers:
| State | Fourseam | Sinker | Slider | Curve | Split |
|---|---|---|---|---|---|
| Bases Empty | 55.8% | 11.3% | 13.9% | 8.2% | 10.8% |
| Runners On | 50.9% | 14.5% | 16.0% | 6.1% | 12.4% |
The first thing to note here: Chen's still thrown the sinker more in pressure situations, albeit not to the extent that he did in his rookie year. Its velocity has increased a bit, though — rising from 91.2 MPH in 2012 to 92.0 thereafter — and with that, it's improved its rate of strikes (64.6%) and home runs (0.6%).
As much as his sinker has gotten better, though, Chen's seen the biggest difference from his slider. Whereas it carried an average velocity of 81.1 MPH three seasons ago, it's gone 83.1 MPH since then, with more vertical movement to match. Unsurprisingly, Chen's accumulated far more whiffs with it (14.4%) than he has with any other pitch; he also owns a 65.3% strike rate and 0.6% home run rate on the pitch. More of these, along with those sinkers, has granted him more effectiveness with runners on base.
Like with Gonzalez, Chen has benefited from some random variation in this transition. A consistently lower BABIP with men on base (despite mediocre hard- and soft-hit rates) has aided his stranding ability, and we shouldn't expect him to sustain that. Moreover, misfortune likely caused much of Chen's first-year sinker struggles; erasing that has certainly played a role in his turnaround.
Nevertheless, there's enough evidence here for me to foresee continued overperformance in Chen's future. I'd certainly take the over on his 71.8% projected rest-of-season LOB%, and would count on him to leave plenty of men on base in the years to come. If the Orioles do re-up Chen, they might have to worry about his early-season velocity, but his play in the clutch shouldn't concern them.
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