Philadelphia Phillies first baseman Rhys Hoskins arrived in the majors on August 10. By August 27, he had 11 home runs, including eight in nine days, and was already being declared the savior of the franchise. But we had seen this story unfold once already this summer. In late April, Los Angeles Dodgers outfielder Cody Bellinger arrived in Chavez Ravine, where he started hitting dingers at a historic rate and basically never stopped.
This isn’t supposed to happen in MLB. Call-ups from the minors often look overwhelmed and out of sorts when facing big league pitching for the first time — if not immediately, then at least at some point in their first month. But that’s not the case this season. Rookies are contending for MVP awards, young hitters across the league are mashing at a ridiculous rate and minor league prospects aren’t missing a beat when earning a first-time call-up.
There’s evidence that this is not necessarily because today’s young hitters are just better than previous years’ crops, but rather because ballclubs are armed with new technologies, which is making them better at determining which players are ready for the majors and when.
This skill is more important now than at any other time in the baseball calendar. Teams gained an additional 15 roster spots on Friday, which they often fill with players from the minors. About 20 percent of call-ups happen in September, and teams usually bring promising prospects up for a preview. That means we get a chance, if only for a brief moment, to see some of the best talent from each team’s farm system.
Hitters called up for the first time1 used to lag behind their more experienced colleagues. Each year from 2009 to 2014, the median batters in this group were typically below replacement level for the season. But since 2015, the same group has posted substantially higher WAR, in terms of both average and median, and these players now easily meet or (more often) exceed replacement level.
Here’s a chart showing the average wins above replacement2 of first-time call-ups by year.
The youngsters have built some of that value with the bat. From 2009 to 2014, first-time call-ups averaged a .218/.278/.336 slash line. An era of low offense didn’t help, but even compared to the league as whole, they were swinging poorly. Since 2015, first-time call-ups have improved to .234/.296/.363.
But the slash lines can be a little misleading. When we look closer, it becomes clear that the post-2015 group of players derives some of its advantage from performing better in the first few months of their major league careers. It seems likely that recent batches of call-ups have been more ready to hit big league pitching when they first arrive, rather than scuffling for weeks as they adjust to higher pitch speeds and better pitcher control.
It doesn’t appear to be a coincidence that this shift happened in 2015. The TrackMan system, which tracks crucial elements of hitting performance like exit velocity and launch angle, started to measure the minor leagues in 2010, but it wasn’t widely used across baseball until 2014 or 2015. Equipped with precise information about how each batted ball was hit, teams can now base their promotions and call-ups on more than just gut feel and minor league outcomes.
This probably doesn’t affect star rookies much. After all, it doesn’t take a fancy tracking system to be able to see that someone like Mike Trout or Bryce Harper is pretty good at baseball and deserves a chance at the next level. Where TrackMan might be most useful is for telling who isn’t up to the challenge of major league baseball. While there are occasional exceptions, batters who can’t regularly muster exit velocities close to the MLB average will struggle to produce offensively. TrackMan offered a way for teams to screen those weaker hitters out. And just as expected, since 2014, the fraction of hitters with below-replacement-level value in their first year in the majors has dropped.
The downside to TrackMan has been that players come in as mature products without the room for growth they once had. Before the advent of radar tracking, players showed a clear learning curve: Between their first 10 games in the majors and their 40th-50th, they gained an average of 92 points of OPS. Since 2015, that improvement has been much more muted, at only 47 points. The performance of the pre- and post-TrackMan era call-ups eventually converges, suggesting that much of the value that recent call-ups produce comes shortly after they join the majors.
All that new information might have revolutionized how teams evaluate hitters, but it is much less useful for gauging pitchers. Hurlers don’t have as much control over how hard the ball is hit, or at what angle. Instead, the equivalent technology for pitchers is probably Pitchf/x, which was adopted in MLB in 2007 and measures such crucial characteristics as the velocity and break of pitches. Pitch tracking came into wide use in the minors in 2009, which coincides with a noticeable bump in rookie production. Young pitcher WAR totals have stayed relatively constant since then, so teams don’t seem to have built on whatever initial advantage Pitchf/x might have brought.
Whether it’s baseball’s first scouting reports or the quantification of a batter’s swing, fresh data has frequently changed the balance of power between veterans and youngsters in the league. As the influx of technology at the major league level makes its way to the minors, the information gap between MLB and Triple-A will start to close. And as the risk that a call-up will flame out decreases, the league might soon be taken over by players in their early 20s making the job look easy.