Marcus Semien, the Quietest Star

Marcus Semien is up to his usual tricks. He’s eighth among all position players in WAR, comfortably the best on a first-place Rangers squad. For the third straight year and the fourth out of five, he’s on track to rack up four-plus WAR as one of the two best players on his team. For someone who didn’t post an above-average batting line until his seventh major league season, it’s an impressive accomplishment.

Perhaps more impressive to me: he’s doing it right under our noses, and no one seems to notice. Semien is good at everything but not in a way that adds up to a tremendous offensive line. His best skill might be durability. He’s clearly a very good player, but his particular set of skills are highlighted by the framework we grade him under. I’m interested in Semien as a player, and I’m also interested in why he’s the poster boy for both what WAR gets right and where it has limits.

Let’s start with how Semien does it. It’s fairly straightforward: he’s above average at every phase of the game. It begins with his plate discipline. To put it simply, he doesn’t make bad decisions about when to swing. In each of the past five years, he’s accomplished an impressive double: chasing fewer pitches than league average and simultaneously swinging at more in-zone pitches than league average. To state the obvious, that’s a great way to both rack up a pile of walks and avoid strikeouts.

If you zoom in on Semien’s decisions, they get even better. He doesn’t have massive raw power; he’s roughly average when it comes to any measure of top exit velocity you care to pick. He delivers extra base hits most frequently when he swings at something over the heart of the plate, more so than whichever slugger you care to compare him to. Guess what? He’s one of the best in the business at doing just that. When pitchers throw him something over the heart of the plate, he swings a whopping 85% of the time. That’s the sixth-best rate in baseball this year, up there with aggressive hitters like Corey Seager and Freddie Freeman.

That might not sound impressive on its own. After all, a few weak hitters dot the top of this list, too. Ezequiel Tovar is third, and he’s far too aggressive for his own good. Joey Gallo and Christian Bethancourt feature prominently at the top of the table. Merely swinging at a lot of pitches down the middle doesn’t mean you have it figured out. But Semien has the other side of the equation under control, too. When pitchers throw him something in the chase zone (not right on the fringes of the zone, but not an automatic take either), he doesn’t swing. Of the 297 hitters who have seen at least 100 chase pitches this year, 243 of them have swung more frequently than Semien. He’s an outlier in both directions.

He needs to be, because as we’ve already covered, his raw power is pedestrian. With middle-of-the-pack exit velocity, you might expect middle-of-the-pack power production. You’d be wrong, because Semien squeezes every last drop out of his contact quality. His park-adjusted isolated power is 16% higher than MLB average so far this year. It was 21% higher than average last year, 61% above average in 2021. By swinging at good pitches, getting the ball in the air, and generally maxing out his batted ball opportunities, he puts up solidly above-average hitting numbers with great regularity.

“Solidly above average” does a good job of describing Semien’s other contributions as well. He’s a plus baserunner — not one of the best in the league but inarguably above average. He’s a solid defender at an up-the-middle position. He’s a scratch defender at shortstop, which makes him a plus defender at second base. No matter the phase of the game, Semien is better than most major leaguers at it.

Exactly how valuable is that skillset? That’s a question sabermetricians still wrestle with. By wins above replacement, Semien has been the tenth-best position player in baseball from 2019 to now. But that statement lacks context. WAR lacks context by its very nature. It’s an attempt to boil down all of baseball into a single number, a goal that inherently removes nuance. Take a look at those aforementioned top ten players, with a few statistics appended:

Semien’s standout skill jumps out: he just plays more than the rest of these guys. His offensive production is above average but not otherworldly. He’s not one of the best shortstop defenders of our generation like Lindor. But he’s out there every single day; he has a whopping 1,000 more plate appearances than Trout, for example. In fact, Semien leads the majors in plate appearances over that span. Freeman is the only other player within 100, and José Abreu is the only other player within even 200.

Why does that matter? Because WAR is a counting statistic, which means that players above replacement level rack up value every time they play (on average). An average player is worth just under two wins over 600 plate appearances — around 1.85 to be more precise. Give that player an extra 130 plate appearances (Semien has averaged 732 in the last three non-COVID seasons), and they’d be worth more like 2.3 wins above replacement.

If you’re a little bit better than average, those extra plate appearances add even more juice. Consider a player who produces 4 WAR per 600 plate appearances. That’s a fringe All Star, more or less — Matt Chapman or Eugenio Suárez in 2022. Turn that up to 730 plate appearances, and that’d be more like a 5 WAR performance. Likewise, Semien finished last year with 4.2 WAR, right in line with Chapman and Suárez. If he’d only been able to play for 600 plate appearances worth of time, though, he’d have finished the year with 3.5 WAR, comfortably behind them.

Should we be giving Semien credit for that extra playing time? It depends on how you view the concept of replacement level. The central concept of replacement level is undeniably clever: If you’re batting for your team, someone else can’t be batting, naturally enough. Value depends on the difference between what you did and what someone else would have done in your place. That concept gets extended to fielding, baserunning, and everything else on a baseball diamond. It’s an elegant way of assigning value, and the general concept of value relative to replacement is pervasive not just in sports but also in pretty much every analytical field you can imagine today. But as Bill James often points out, the real sticking point is in that definition of replacement level.

WAR handles this question more or less by fiat. Or, well, fiat isn’t quite right. In 2013, FanGraphs and Baseball Reference agreed to use the same replacement level value in their calculations of WAR. That value works out to 1,000 wins above replacement per year, split across the entire major leagues. That’s based on a variety of studies that estimate production by freely available players, but it’s just that: an estimate.

For the most part, WAR being slightly off just isn’t that big of a deal for single-season purposes. If you calculate replacement level wrong, so what? Most full-time starters get 600–650 plate appearances a year, so the players we’re comparing when we look at WAR leaderboards are putting up their statistics over similar workloads. One player is usually way out in front, anyway; Semien could have batted 1,500 times last year without putting up as much value as Aaron Judge.

Semien is right on the borderline of making me reconsider the argument, though. He plays so much that the playing time differences really do matter. He’s worse on a per-plate-appearance basis than everyone in his general vicinity on the WAR leaderboards. It’s absolutely true that the best way to calculate value is relative to what you’d be getting from a replacement. The definition of replacement merely matters more for Semien.

Consider this: if we look at wins above average instead of above replacement, Semien drops to 15th instead of 10th. Notably, he’s behind Fernando Tatis Jr. despite having batted more than 1,000 times more often over that window. Instead of being a rounding error behind Trout, he’s 5.5 wins behind. Change the baseline, and value changes right alongside it.

Which baseline is correct is a trickier question, and in my opinion, value is ultimately heavily dependent on team context. Good teams have better replacements than bad teams in general. Last year, for example, the Dodgers had to dip into their replacements when they faced a wave of injuries. Their replacements were great, though, producing 2.5 WAR in 528 plate appearances because Trayce Thompson, who they acquired for nothing in the middle of the year, put up 2.8 WAR. The Astros, on the other hand, got 0.3 WAR out of replacement level players over 708 plate appearances.

Who your direct replacement is matters a lot, in other words. The problem with calculating that for Semien is that he mostly hasn’t had a replacement. He played in 161 games last year. The year before that, he played in all 162. He played in all 162 in 2019 as well. My best guess is that Josh H. Smith would get extra reps if Semien played less, but it’s really just a guess.

Smith looks like a roughly average player to me. Our projections think he’ll be worth 0.6 WAR in 175 plate appearances the rest of the way this year, which works out to around 2 WAR per 600 plate appearances. But if it’s not Smith as a replacement, it might be Travis Jankowski (via some infield/outfield switches), and we think he’s much closer to replacement level. In other words, the context of replacement level isn’t even clear when we pick a specific team and a specific year.

Where does that leave me in thinking about Semien’s value? I’m inclined to side with our calculation of WAR rather than trying to recalculate some higher replacement level, even after presenting the argument against it here. The truth is that durability is valuable. Semien’s extra playing time lets the team give other players rest and use situationally useful guys when they’re at their best rather than overexposing them. His durability is truly impressive, too, which makes me more inclined to give him credit for it. Excluding 2023, he has eight pieces of black ink on his Baseball Reference page — games in 2019, plate appearances in 2019, games in 2021, plate appearances in 2021, games in 2022, plate appearances in 2022, at-bats in 2022, and sacrifice flies in 2022. In other words, he’s the very best in the league at being out there every day, and providing an above-average rate while doing so.

Could you argue that WAR slightly overstates Semien’s value? Sure. But I bet you his teammates would tell you otherwise, and I bet you teams would as well. The plate appearances we don’t see from his replacement might be impossibly tricky to value, but that doesn’t mean they’re worthless. It just means our tools aren’t perfect, and while I think we’re pretty good at valuing production these days, I don’t think we’ll ever get all the way to perfection.

Statistics in this article are current through games of Saturday, June 17.