Marcus Semien is having a moment. Semien finished the 2018 season with 3.7 WAR, nearly doubling the next-best total of his career. There was a catch, however, in that Semien’s WAR was largely fueled by a sudden jump in his defensive numbers. While the numbers for Baseball Info Solutions and Ultimate Zone Rating never completely agreed on Semien, they both thought he was a below-average defender at short. Coming into 2018, per 1350 innings, BIS had Semien at -2.7 runs, while UZR had him at -7.6 runs. In 2018, those numbers were +8.5 runs and +7.9 runs respectively, numbers that, if believed, meant that Semien had added 10-to-15 runs of value from somewhere very unexpected.

While a White Sox prospect, it was thought that Semien would likely struggle playing shortstop in the majors. In the minors, he made 60 errors in 2203 innings (37 errors per 1350 innings), which led the White Sox to find as much time for him at second and third base as possible. And while error rates usually come down in the majors, Semien committed 35 errors in his first season as the A’s shortstop. You could argue that errors can be overused as a method of defensive evaluation — and you’d be correct — but there are limits. UZR actually had Semien as slightly above-average in range, with the -11.7 total run estimate coming from a brutal -12.6 runs from errors. That’s not a run-of-the-mill error-prone season, either:

Looking at the top error seasons, you see a lot of players who outright lost their job at shortstop over the next few seasons. With Franklin Barreto believed at the time to be Oakland’s shortstop of the future but not yet ready to assume the role, the A’s decided to stick with Semien and get him some help at the position. The A’s hired Ron Washington to assess Semien and they got to work. Ken Rosenthal wrote about Semien’s defensive transformation last month:

“He didn’t know how to play shortstop,” says Washington, who was a major-league infielder in 1977 and from ’81 to ’89. “He would get to balls. But he didn’t know how to get to them where he would be in position to finish the play. He just used his athletic ability with no technique. “I saw it in the beginning. I told him. And we went to work.” Washington instructed Semien to use a larger glove, open his hand to create more surface area and focus on catching the ball in the pocket. The way Semien had been fielding grounders, the ball would often get jumbled up in his glove and not even be secure in his hand before he made throws.

Defensive numbers are volatile, so having a second year of improved defensive numbers significantly betters the chances that Semien’s reinvention with the glove is for real. That the improvement is largely driven by error rate is an even more promising development because though errors themselves aren’t a great measure of defense, error runs tend to be more predictive on a year-to-year basis than range runs. This isn’t surprising given that range numbers necessarily have to jump into evaluating theoretical plays that never happened. In error runs, Semien’s +4.6 ties with Paul DeJong for the best in baseball at any position in 2019. If he continues on this pace, he will have added roughly 20 runs compared to the 2015 season, simply from avoiding errors.

On pace for a six-WAR season overall, defensive performance alone can’t explain Semien’s 2019. As a starter in the majors, Semien had settled into a fairly predictable pattern: you could pencil him in for a .250/.310/.410 season and rarely be that wrong. Never finishing with an wRC+ below 90 or above 100 (at least since his cup of coffee in 2013), Semien has spiked a 115 in 2019 with a .271/.350/.449 line. All three of those triple-slash numbers would comfortably be career highs. He’s also not doing it the traditionally flukey way of having an unsustainable BABIP (he’s below his career average) or in the hot 2019 way of crushing every low pitch he sees into the seats (he’s hitting more grounders than he ever has). Semien’s improvement can largely be tied to improvements in his plate discipline.

In the space of two seasons, Semien has cut more than 40% of his strikeout rate, something that’s not easy to do as a veteran. This is also no accident, as Rian Watt discussed back in early June. One of the traps of plate discipline is when it results in being too passive at the plate, a problem that contributed to the truncated careers of Ben Grieve and Jeremy Hermida. But what Semien’s been successfully able to do is use plate discipline as a means to an end; the purpose isn’t drawing more walks by design, but instead, trying to get pitches he’s more comfortable hitting. Semien’s pulled off the trifecta of swinging at fewer bad pitches, hitting a lot more of the ones he does offer at, all while hitting the ball harder:

The effects of Semien’s improvements can now be seen fully in his long-term projections. Coming into the season, ZiPS saw him hovering around two wins over the next several years before age-related decline dragged him down. The crystal ball saw a three-year, $43 million contract awaiting Semien in free agency after the 2020 season:

ZiPS Projections – Marcus Semien Year BA OBP SLG AB R H 2B 3B HR RBI BB SO SB OPS+ DR WAR 2020 .259 .329 .422 595 85 154 33 2 20 76 64 114 13 105 4 3.6 2021 .261 .331 .432 560 81 146 32 2 20 74 60 104 10 108 3 3.5 2022 .257 .328 .431 540 77 139 30 2 20 71 58 100 9 107 2 3.2 2023 .254 .322 .419 520 72 132 28 2 18 65 54 93 8 103 2 2.6 2024 .251 .316 .406 495 65 124 25 2 16 59 49 85 7 98 1 2.1 2025 .244 .307 .383 467 59 114 22 2 13 53 43 74 6 89 0 1.3 2026 .239 .296 .366 435 51 104 18 2 11 45 37 64 5 81 -1 0.6 2027 .231 .282 .334 350 38 81 13 1 7 33 26 47 3 70 -2 -0.2

ZiPS buys the evolution of Semien, now projecting him to plate at near All-Star levels for three seasons, enough to boost a projected post-Oakland contract into the $20 million per season range for a few seasons.

One of the most difficult things for a veteran to do is to fundamentally change themselves as a player. Through an immense amount of hard work, Marcus Semien has transformed himself both at the plate and in the field, turning himself from a stopgap shortstop to a key reason the Oakland A’s are currently only a game and a half out of a playoff spot.