We’ve officially entered extension season. This happens every year around the start of spring training, with arbitration hearings ending and with opening day coming up. Some expect that this particular extension season will be unusually busy, given the concerns players have about the state of the free-agent market. Aaron Nola just signed an extension with the Phillies. Jorge Polanco just signed an extension with the Twins. And Max Kepler has also just signed an extension with the Twins. Nothing against Polanco, but I find the Kepler move more interesting.

Kepler was already looking at a 2019 salary of $3.125 million, in the first of four arbitration years. He’d qualified as a Super Two. That’s wiped out now, with Kepler and the Twins agreeing to a five-year contract worth $35 million. There’s also a sixth-year club option, worth $10 million. Kepler, therefore, has signed away up to two years of would-be free agency. From Kepler’s own standpoint, he’s now guaranteed his own long-term wealth, as a German kid made good. He wouldn’t have agreed to this if he weren’t happy to do so. At the same time, you wonder what could’ve been. Where is Kepler going to be, as a player, a year from now?

For a while, it’s felt like Kepler has made regular appearances on lists of breakout candidates. He was a good prospect, and he’s been a fine player, posting 5.2 WAR through age 25. The breakout, though, hasn’t yet happened. His career wRC+ is 94. Last year’s wRC+ was 97. That’s not bad, for a quality gloveman. It’s also not good. Kepler the hitter has been incomplete.

It’s easy to grow impatient. It’s easy to have grown impatient with a number of Twins players, for that matter. But, thinking about Kepler today, I’m reminded of this Dan Hayes article from early December. Teams continued to call the Twins, inquiring as to Kepler’s availability. The Twins never formally put him in a trade. Other teams still see a breakout candidate. The Twins still see a breakout candidate. There seems to be something of an industry consensus, and it’s not a tough thing to explain.

Kepler is newly 26 years old. So far, through about three seasons, he’s been worth 1.9 WAR per 600 plate appearances. He’s an above-average baserunner, and he’s a terrific defensive outfielder. DRS has liked him in right field and center. UZR has liked him in right field and center. And I also have three years of Statcast’s Outs Above Average. None of these measures are perfect, of course, but among outfielders with at least 1,000 innings since 2016, Kepler’s Outs Above Average per 1,000 innings ranks him in the 88th percentile. Among outfielders with at least 2,000 innings since 2016, Kepler is in the 85th percentile. Some names on either side of him: Jackie Bradley Jr., Jason Heyward, Manuel Margot, and Leonys Martin. Kepler’s great in the field, so it’s probably not a coincidence his contract looks a lot like Ender Inciarte’s, from December 2016. Inciarte was also Super Two, with a similar profile. In that sense, the terms are fair, in that there’s precedent.

But, let’s look at Kepler’s upside. Let’s look at some gains he already made in 2018. He lowered his ground-ball rate, or, if you prefer, he raised his launch angle. Importantly, Kepler increased his walks while also trimming his strikeouts. A chunk of that came from improvement against left-handed pitchers. In terms of K-BB%, eliminating intentional walks, Kepler improved by 7.8 percentage points. Between 2017 and 2018, there were 246 players with at least 250 plate appearances in each season. Kepler’s improvement in K-BB% tied for the fourth-biggest in baseball. He was practically right there with Alex Bregman.

Going hand-in-hand with that, Kepler swung less often out of the zone. He swung more often inside of the zone. His contact rate shot up, to something well above average. When you look at how Kepler’s peripherals moved, you’d think he just had his emergence. Yet his wRC+ just inched from 93 to 97. Kepler stands 6’4, and he has legitimate power. There ought to be more in the tank.

I don’t know if there’s a clearer way to illustrate this than by looking at player comparisons. This is the very essence of what makes Max Kepler so intriguing. For all regular player-seasons since 2015, I gathered K-BB%, average exit velocity, average launch angle, and wRC+. For the former three measures, I calculated the absolute values of z-scores, using Kepler’s 2018 season as the comparison point. In other words, I calculated the differences in standard deviations. I added up the three z-scores to come up with something I call the Comp Score. Here’s Kepler’s 2018, and its 20 closest recent comps:

Anything jump out? Other than Kepler’s 2018, every single wRC+ is north of 100. Other than Kepler’s 97, the lowest wRC+ in there is 109. The average wRC+ for the ten closest comps is 133. For the 20 closest comps, it’s 130. For the 30 closest comps, it’s 131. This isn’t exactly why teams have kept calling on Kepler, but this is *basically* why. When you see Kepler’s blend of discipline, pop, and launch, you think he should be a quality hitter. You think that, because everyone else has been a quality hitter.

Now, it’s important to point out it’s not just all luck. Bad luck isn’t the reason Kepler hasn’t had a breakout yet. Let’s look at another Statcast metric, called Sweet Spot%. It’s the rate of batted balls hit between +8 and +32 degrees above the horizontal. In this table, you’ll see Kepler’s three seasons, along with the average Sweet Spot% for hitters with similar average launch angles:

Max Kepler, 2016 – 2018 Year Launch Angle Sweet Spot% Cohort% 2016 8.3 30.1% 32.0% 2017 12.7 31.9% 34.4% 2018 16.2 30.5% 36.7% SOURCE: Baseball Savant Sweet Spot% is the rate of batted balls hit between 8 – 32 degrees. Cohort% is the average Sweet Spot% for players with comparable launch angles.

Kepler hasn’t yet made progress here. He hasn’t hit as many balls square as you’d expect, and those sweet-spot batted balls are the most dangerous ones. Take a few away, and the overall numbers turn south in a hurry. As Kepler’s overall launch angle has increased, his hasn’t increased his own rate of batted balls hit at the ideal angles. That’s why his productivity hasn’t yet surged. That’s also something we should expect to regress toward the average, moving forward. I’m not going to throw another plot or table at you, but according to the simple math I’ve run, Sweet Spot% over- and under-achievers have historically regressed toward the mean by about 50% in the following season. Kepler might again under-achieve in 2019, but he should at least under-achieve by less, quickly driving his numbers up.

Max Kepler has already improved. He’s improved in just about every way, but the big one. Yet with those gains in 2018, he’s laid the foundation for a surge in the future, as he takes another half-step or so forward with his bat control. With just a few more line drives and well-struck fly balls, Kepler will look like a dangerous hitter, who can also run the bases and play the hell out of the outfield. Put it all together and you’ve got legitimate star potential. You could have a player with a 3-WAR baseline. Maybe 5- or 6-WAR upside. The skillset is there. It seems like it’s coming together.

It just hasn’t come together yet. Not all the way. Which is why Kepler was willing to sign for these terms. You can also see why the Twins would be ecstatic to have him signed for these terms. From where the Twins sit, they understand Kepler’s potential. From where Kepler sits, he knows he’s not a finished product, and he can’t take his own improvement for granted. In every single individual case, you can see why the player agrees to a long-term extension. Who could turn down that kind of guaranteed security? If Kepler were a gambling man, though, he could’ve taken his 2019 salary, and bet on himself to get better. Then he could engage in talks about an extension, after having a breakout season. The breakout isn’t a lock. No breakout is ever a lock. It’s just that the odds are looking favorable. That’s why the Twins came at Kepler, just as the Indians came at Jose Ramirez. One of the most jarring realities of the current era is that oftentimes teams know more about the players than the players themselves.