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Marcus Semien dramatically improved his defense

A’s fans used to hold their breath when the ball was hit to short, but in 2018 they could rest easy.

MLB: Oakland Athletics at Minnesota Twins Brad Rempel-USA TODAY Sports

The Oakland Athletics vastly exceeded their preseason expectations in no small part because of breakout years from Matts Chapman an Olson, Jed Lowrie continuing his mid-30s renaissance, and exceptional performances from their bullpen. Those years have overshadowed the step forward taken by Marcus Semien.

Semien’s year at the plate looked like a typical season for him. He cut down on strikeouts by about four percentage points from his career average, but that’s the only notable improvement or change. His .255/.318/.388 slash line was about what was expected and his 96 DRC+ matched his career mark.

Still, Semien posted a career-high WAR no matter which iteration of the stat you use. By WARP, Semien ranked fifth among all shortstops at 5.1. In Semien’s previous three seasons, he had averaged 2.5 wins, so why the three-win jump? Semien’s defense vastly improved in 2018.

Since joining Oakland, Semien has cost the A’s more runs than he’s saved. Between 2015 and 2017, Semien accumulated -10 DRS and he averaged -7.9 UZR/150. In 2018, his DRS jumped to 9 and his UZR/150 rose to 6.4.

Susan Slusser of the San Francisco Chronicle wrote about how many of Semien’s improvements on defense were the result of his working closely with Ron Washington.

“I wasn’t even holding my glove correctly,” Semien said. “Wash had to tell me to spread my hand to give the glove the widest surface, and then give the glove a push through the ball to eliminate any funky hops.”

In addition to how he held his glove, Washington helped him improve his footwork and how he set his body before he threw. Before 2018, Semien had a tendency to close his body off from his target, adding in extra movement as he tried to complete a throw.

These two screenshots were taken on similar plays, routine grounders to Semien’s right. In each instance, Semien had time to set his body to make the throw. In the first, which was taken in 2016, Semien made a poor throw wide of his target.

Notice how Semien’s front shoulder is closed off toward first base. His body is twisted so that he’s looking over his shoulder toward his target.

In the second, taken in 2018, Semien completed the play with an accurate throw.

On the more recent play, Semien keeps his front shoulder pointed toward his target giving him a clear view of where he wants to throw and eliminating extra movement as part of his throwing motion.

Angling his body more efficiently helps reduce throwing errors on routine plays. Those throwing errors alone aren’t worth much over the course of a season. They’re certainly not enough to account for an 18-run difference from 2017 to 2018. Regardless, any improvement on defense is welcome, no matter how negligible the run value.

What’s more important to Semien’s breakthrough is his increased range. FanGraphs tracked his range factor at 8.2. His previous high was just 0.6. He also made 133 out of zone plays which is 27 more than his previous mark. Looking at spray charts for his made plays, it looks like Semien has become more comfortable catching pop flies to short left.


No doubt Semien switching to play in short right on the overshift instead of behind second base helped out his defensive numbers as well. It makes sense that a player who struggles with arm accuracy should be taking the shorter throws.

As long as Semien can continue his defensive adjustments and keep hitting the way he’s had throughout his career, he could be around around a four-win player for each of the next few seasons. At $3.1 million, Semien is a steal for the A’s and he’s a major reason why their future looks bright.

Kenny Kelly is a writer for Beyond the Box Score and McCovey Chronicles.