Jeff Niemann's overall stat line from last year is misleading. The conventional numbers suggest that he had a solid year, starting 29 games and compiling a 4.39 ERA and 12-8 record in 174.1 IP. The most advanced numbers aren't all that different, with xFIP and tERA both at 4.35 and his FIP at 4.61 because of a high HR/FB% of 12.2. A look at his month by month splits, however, might paint a more familiar picture.
Niemann's ERA absolutely ballooned in August and September. Perhaps not coincidentally, Niemann went to the DL after his start on August 3rd. It would be his last start until August 25th when, against the Angels, he would give up 10 earned runs in 3.1 innings. His next start wouldn't be much better when, against Toronto this time, he was obliterated for 7 runs in 5 innings. The murmurs from fans and the media alike were that Niemann had been rushed in his return to the rotation and clearly wasn't ready for his return. And yet, we must always be careful not to mix correlation and causation, even if the relationship is perhaps logical.
First consider Niemann's peripherals before and after the injury. Peripherals are generally more stable indicators of talent than things like ERA.
|Before Injury (578 PA)
|After Injury (155 PA)
*For those of you unfamiliar, SIERA is a defense independent pitching stat in the same vein as tERA. It's similar to xFIP but it factors in LD% as a controllable skill. It's roughly equal to xFIP in terms of predicting future performance, but is used here instead of xFIP (normally my stat of choice) because it is PA based.
We see here that Niemann's strikeout rate actually increased during this time. His walk rate increased drastically, however, and his ground ball rate suffered a precipitous decline. The loss in these two aspects both caused his SIERA to rise, albeit not drastically. Although much of the decline was due to a shift in "luck"-based stats (HR/FB was 8.5% before and 9.2% after, BABIP was.252 and .320 after), Niemann's SIERA did indeed rise by a quarter of a point, which makes a difference of half a win over the course of a year.
The logical next step is to investigate whether this decline might be the result of a change in pitching. Consider Niemann's pitch velocities:
Every indication is that Niemann's stuff was as good (if not better) than it was pre-injury. Considering that his strikeouts actually increased, it's also probable that his stuff was still just as effective (perhaps moreso). Given that his walks and fly balls increased, however, a decrease in command may be a more likely cause. Consider how Niemann's Zone% changed.
It seems as though Niemann's Zone%s are fairly similar for his fastballs and he simply doesn't have enough pitches with his off-speed stuff after his injury (27 changeups, 99 curveballs, 34 splitters, 52 sliders) for those to be too meaningful. Additionally, Niemann's off-speed pitches didn't show a uniform trend with changeups being out of the zone more often and the rest being in the zone more often. Although there's more to command than placing the ball in the zone, it would be extremely difficult to attempt to quantify command in a more effective way. At the very least, it appears as though Niemann didn't have a harder time keeping the ball in the zone.
Niemann's outstanding apparent performance leading up to August 3rd, particularly his 3.12 ERA and 10-3 record, was clearly unsustainable. Perhaps because of this, his horrific post-injury starts seemed an even starker contrast to his remainder of the year numbers. Although his peripherals declined slightly, this appears to be the result of random variation more than a decline in talent (although this is hard to state objectively). His stuff and ability to place it in the zone are the same quality they were before, at least. As a result, it's probably safe to say that Niemann's still a low-4s ERA true talent type of guy who will put up ERAs in the high-3s as a result of good defense; I certainly wouldn't sleep on him.