A little more than two months ago, I noted that the Mariners’ bullpen had been — through that point in the season — among the most improved in the game, relative to preseason expectations. Before the 2018 season began, we projected Seattle relievers to strike out 9.1 batters per nine innings and walk 3.5.

A couple weeks into the season, they weren’t doing that. They were performing much better than that, actually. At the time I wrote that first article, Mariners relievers had struck out 10.7 batters per nine innings and walked just 1.5, both of which were tremendously good numbers and perhaps merited further investigation at the time. But it was still early, so I kept the article general and cautioned that we shouldn’t necessarily expect the men in teal to keep up their early-season performance too much longer.

Well, we’re now more than a third of the way through the major-league schedule, and the Seattle bullpen has stayed improved. In fact, the Seattle bullpen has been among the top three or four in the game, no matter which way you slice it, but this piece is about improvement against expectations. Here’s an updated version of a chart I included in my original article, which plots each teams’ actual relief K/9 and BB/9 (adjusted so that positive figures are good in both cases), through games played on Friday, against our preseason expectations of the same:

Observant readers will note that there is another happy story to tell here about the Astros’ pen — featuring Héctor Rondón and Chris Devenski — and a sad one about the Orioles, featuring almost every Oriole. But, again, this article is about the Mariners, whose improvement relative to projections in both K/9 and BB/9 has been outstanding — and key to the club’s outrageous performance in one-run games, which was detailed here by my colleague Jay Jaffe. Maybe it’s finally time to dive a little bit deeper into what they’ve been doing and why it’s been successful.

It’s often helpful to think of team improvement (or decline) in any given area as the product of two separate dynamics: the improvement (or decline) of returning players, and the change in quality between new players and those who they replace. In the Mariners’ case, it’s probably been a little bit of both. Here’s a chart for your consideration, which lists the top ten Mariners of 2018 by the percentage of the team’s relief innings they’ve thrown (i.e. not including Wade LeBlanc’s gem Saturday night), through games played on June 15, and also their 2018 FIP in relief. It then does the same for 2017:

I’d like to first draw your attention to five of the names on the left-hand list: Nicasio, Bradford, LeBlanc, Cook, and Elías. Those five men, each of whom joined the Mariners from other clubs at the start of the 2018 season, have thrown a combined 38% of the Mariners’ relief innings this season, and have done so with a combined 9.3 K/9, 2.0 BB/9, and 3.63 FIP. The men they replaced on the list (Pagan, Zych, Rzepczynski, Cishek, and Gallardo) threw 28% of last year’s innings, with a combined 8.01 K/9, 3.58 BB/9, and 3.87 FIP. The new Mariners have been better than the old Mariners, and they’ve thrown a higher percentage of the team’s innings, too. (It’s worth noting here, perhaps, that Nicasio has been out since June 6th — although he does appear to be close to returning.)

But the holdover Mariners aren’t bad, either. Last year, Díaz, Pazos, Vincent, Altavilla, and Lawrence threw 48% of the team’s relief innings, with a combined 9.9 K/9, 3.4 BB/9, and 4.00 FIP. (Vincent and Altavilla have also recently been out. Vincent appears near return; Altavilla does not.) So far this year, they’ve thrown 49% of Seattle relief innings — almost the same proportion, albeit in a different internal ratio — and have a combined 10.9 K/9, 3.2 BB/9, and 2.89 FIP. The new Mariners are better than the old Mariners, yes, but the holdover Mariners are much better than they were last year.

So it’s a little of both. But because the new Mariners’ performance doesn’t look that different from their performance for other clubs last year (Nicasio, Bradford, LeBlanc, Cook, and Elías had a combined 3.66 FIP in relief for other teams last year, which is only marginally better than this year’s 3.63), let’s just chalk that improvement up to good scouting by Jerry Dipoto and the M’s front office. But what’s happened to the holdover Mariners to improve them so dramatically from one year to the next? Four words seem relevant here: Edwin Orlando Díaz Laboy.

Last year, the four non-Díaz holdovers posted a 4.00 FIP over 207 innings pitched. This year, they’ve posted a 3.48 FIP over 75. That’s a definite improvement, to be sure, but it’s nothing compared to Díaz’s jump from 4.02 to 1.98. Díaz went from being big and “projectable” to, in the words of FanGraphs resident Jake Mailhot, “nearly unhittable.” That’s not a turn of phrase, either: Díaz has struck out more batters, as a percentage of all those he faces (41.3%), than all but four other pitchers, and his swinging-strike rate generated (19%) is better than all but two.

A lot of that success has been driven by, in Jake’s words, Diaz “finally learning how to pitch rather than just throw.” His release point has become much more consistent this year — somewhat lower — and he’s taken advantage of the natural arm-side run on his fastball to make it a much more effective weapon against righties and lefties alike (inside to the former and outside to the latter). As Jake also notes, this improved use of his fastball has also driven improvements in his slider, which now looks a heckuva lot more like the fastball at the batter’s decision point than it did before. Both pitches have become more valuable on a rate basis this year despite not changing much in velocity from 2017.

The Mariners are having what already looks now, at the outset of a long summer, to be one of those special seasons that grabs a fanbase by the heart and reminds them why they devote so many of their afternoons and evenings to this game. There have been a lot of reasons to watch the Mariners this year, but my favorite has been the bullpen. The new guys are good. The old guys got better. The whole crew is among the best in the game. The Mariners’ bullpen started strong, and it’s stayed that way, despite injuries. Now that Nicasio and Vincent are on their way back? Bring on the summer.