Marcus Semien is the reason the Athletics will make the postseason

No team is a single player, despite the flashy headline. A lot of things had to go right for the Athletics to be projected to win 98 games, an astonishing feat considering they lost Sean Manaea to injury for most of the year and didn’t make substantive improvements over the offseason.

Naturally projection systems thought they’d regress a small amount, and yet, to everyone’s surprise, they will likely finish with a nearly identical record to last season. Now, for that to happen, there have to be quite a few surprises and bright spots, and the team had its fair share.

Liam Hendriks, chief among them, was a revelation as a new closer, putting up a 1.64 ERA in 82 ¹⁄₃ innings. Matt Chapman was his usual self despite a slightly diminished offensive profile, putting up yet another five-plus-win year. While the rotation was adequate, midseason additions like Tanner Roark and Homer Bailey gave them an all-around solid pitching depth. None, though, were more shocking than Marcus Semien.

Semien represented, at one point, one of the many things wrong with the White Sox organization. Brought into the club at a time when they were supposedly on the rise, adding talents like Chris Sale, José Quintana, Adam Eaton, and others, Semien was supposed to be one of them; he was listed as a top-100 prospect by Baseball America before the 2014 season.

He put up a collective 90 OPS+ with the Sox, ultimately flipped to the A’s in the failure of a Jeff Samardzija trade. It was only cruel irony that he spent years languishing, only becoming the very player that the White Sox could have used to contend way back then.

It wasn’t like his tenure in Oakland was all roses; in fact, most of his A’s career looked remarkably similar to Chicago, hitting to a 98 OPS+ up until last season. Yet one of his transformations happened last year, and one more in this one, to make him the complete player that made him a full-on star.

The first transformation was primarily defensive. Semien had never done better than 1.8 Defensive Runs Above Average via FanGraphs, nor better than 2.7 FRAA via Baseball Prospectus. In 2018 it was 15.6 and 16.2 last year, respectively, and this year just a bit worse at 13.2 and 3.3. The reason for this is not his ability to make routine ones, but those considered mid-range by Inside Edge. If you scale his buckets of plays to 1, and see how he improved year-to-year and adjusted for innings, you’ll see this visualized:

Ultimately this shows the limits of defensive metrics; while his routine plays have hovered around 95% completion rate, basically a handful of mid-range plays made convert nearly straight into runs above average, giving him a seemingly a massive boost in defensive value. The result is a league average offensive performance in 2018, but 3.9 fWAR.

Because, according to Statcast, most of these plays were in the mid-range area, a lot of the reason this was successful was positioning, as the A’s in the last two years positioned Semien closer to the hole in between the bags...

...that means that the range to his left is certainly better than the range to his right, so glove-side plays are easier on balls that are seemingly about to fall behind second base.

The second revolution was offensive, and that basically took all the way until this year for it to fully bloom. Semien had mostly been a 90-100 OPS+ player, and now he is hitting .283/.364/.516, which is also at an All Star level.

What’s funny about him is that he always had the high launch angle, and it’s not like the Athletics just discovered that trick; it has almost always been over 13 degrees. He has also had a hard-hit rate over 30% with a very similar xwOBACON, so it’s not like he’s not drilling the ball when he makes contact. It’s essentially the amount of hard contact, where he has nearly doubled the number of barrels from 23 to 44. The answer to how he did that largely comes down to plate discipline:

The classic formula for The Improving Player is always fewer swings at pitches outside the zone, thus fewer strikeouts and swinging strikes. And because he largely was making decent contact already, you add a few more barrels and a few more walks and you get a very above-average performance. When you add in the defensive improvements, you get a jaw-dropping seven-win season.

Is it going to last? Probably not. ZiPS pegs him at a 111 wRC+ moving forward which seems altogether fair given the history and the profile, and it’s not like we saw whopping changes in his overall exit velocity. The defense is certainly improved, but this is the prime time for the aging curve to work its magic so Oakland will have to continually get creative with positioning, considering it seems this is effective for a certain level of difficulty of a play.

Yet you take all of it into account and it is still seven wins the Athletics may not have seen; there’s some alternate reality where he is replacement level and they’re staring down the barrel of a postseason elimination. Plenty went right for this team, but Semien was the most right of them all.