Last season, there were 129 relievers who threw at least 50 innings. Cody Allen ranked 12th in strikeout rate. Tyler Thornburg was one slot ahead of him. Craig Kimbrel ranked 15th in K-BB%. Tyler Thornburg was one slot ahead of him. Ken Giles ranked tied for 20th in adjusted FIP. Tyler Thornburg was one slot ahead of him.

Thornburg didn’t draw a lot of attention, having a breakout year in a crowded bullpen on a go-nowhere Brewers team. He’s now become professional property of the Red Sox, him and his three years of arbitration eligibility. While Thornburg might’ve been off the general radar, he’s a big addition as a controllable setup guy for a team that wanted to make its bullpen more dominant. In return for an underrated power righty, the Brewers are getting their own underrated package. Dave Dombrowski has dipped into his farm again, and other front offices like when he does just that.

Going to Boston:

Tyler Thornburg

Going to Milwaukee:

We’ll get to the Brewers part later. Thornburg is the headliner, so we’ll focus there first. He’s going to set up for Kimbrel, easing the potential burden on Joe Kelly. At some point he could and should be joined by Carson Smith, and so if all goes well, the Red Sox’ late-inning relief will resemble a strikeout machine. The Sox could have Thornburg for three years, and this falls in line with the theme of contenders trying to assemble deeper, more dominant bullpens.

Getting Thornburg is all about focusing on 2016. Prior to that, he didn’t throw enough strikes, or he didn’t miss enough bats. Maybe both; I’ll leave that decision to you. But the strikeouts just leaped forward, and for that Thornburg could credit sustained health and a better, sharper curveball. He’s always had a changeup and a rising four-seamer that gets into the mid-90s, but — the following images come from Baseball Savant. On the left, Thornburg’s curveball locations from 2015. On the right, his curveball locations from this past season.

Thornburg was better than ever at locating swing-and-miss curves below the zone. As such, it’s no coincidence his strikeouts took off, as the 12-to-6 curveball is half of a classic pair with the rising heat. Hitters can’t easily protect against fastballs up and curveballs down, and there were simply fewer curveball mistakes. As long as these gains keep up, Thornburg should remain greatly effective.

It helps, too, that for his career, Thornburg has been even better against lefties than righties. That’s in large part a function of his over-the-top delivery, but that makes him less exploitable by pinch-hitters in high-leverage appearances. There’s nothing wrong with a lefty or righty specialist, but Thornburg could reliably get three or four outs, no matter who’s at the plate. It’s one less way for the Red Sox to be vulnerable, one fewer thing for John Farrell to worry about.

The Thornburg profile looks good. His numbers were great. You love the team-control aspect. On the other hand, long-term relievers aren’t exactly as dependable as long-term position players. The Red Sox can’t expect to get a certain contribution in 2019. Thornburg has already had some issues with his elbow. And he also wasn’t acquired lightly. Thornburg is the third late-inning reliever the Brewers have moved for a substantial return in the last several months, and this advances their rebuild, even if they remain a long ways off.

Travis Shaw is the familiar name. He’ll be the Brewers’ new third baseman, and as unexciting as he is, he’s 26, and they could have him for five years. He’s been worth exactly 3 big-league WAR in 778 plate appearances. He’s looked more or less like a league-average player, and though he struggles against lefties, and while he was somewhat exposed with more playing time, and while he also has done most of his hitting in Boston, a player like this has value just for purposes of deepening a lineup. Shaw is legitimately useful, right now, and he’ll cost the Brewers almost nothing. He’ll probably give them about the same WAR as Thornburg would’ve.

Josh Pennington is less familiar, and he has yet to pitch above low-A. Even there, his performance wasn’t terrific. He’s a long-shot, but he’s also a quick riser, having come back from surgery throwing consistently in the mid-90s. Pennington became one of the Red Sox’ better pitching prospects, and even if he can’t keep it up as a starter, he has a power repertoire that would be easy to imagine in relief. There’s something there.

And yet Mauricio Dubon is the piece I like the most. It doesn’t take a lot of investigation to figure out why — he’s a 22-year-old shortstop who just ran a 151 wRC+ in a half-season in Double-A. By the numbers, Dubon looks promising, and there are signs of very encouraging growth.

Not even that long ago, Dubon profiled as a slap-hitter. In 2015, he ran a ground-ball rate of 56%. In high-A at the start of 2016, that rate was 52%. After Dubon moved up to Double-A, that rate dropped to 43%. Not only were there more balls in the air, but pretty good power followed. You can see it in the ISO, and you can see it in the locations. Two years ago, Dubon pulled 20% of his batted balls that went to the outfield. This past half-season, he pulled 36% of his batted balls that went to the outfield. In short: Dubon put balls in the air, and he increasingly directed them to left. After a 2015 in which he finished with 29 extra-base hits, he just had 32 in Double-A alone, in half the time.

If it sticks, that’s great. If it doesn’t, welp. Sometimes prospects do have random spikes, and if Dubon regresses, he looks more like a utility sort. But, for example, Aledmys Diaz recently had a sudden power spike in the minors that he translated into the majors, allowing him to come out of nowhere. Dubon is now a legitimately exciting prospect, one more young guy for the Brewers to expect to debut in the nearer-term future.

From the Brewers’ standpoint, this is almost a no-brainer. You never know when Thornburg could regress or break down, and strikeout relievers presently have very high value. They’re not really to be considered long-term assets. From the Red Sox’ standpoint, Thornburg is really good, right now, and he fills a gap. The Red Sox are trying to win a World Series as soon as they can, and Thornburg makes them deeper. All of the very best prospects are still in the system. But there is a good amount of value here changing hands.