The Mariners find themselves in something of a tight spot this offseason, don't they? Sure, they won 89 games last season, their most since 2003, but that doesn't make them an 89-win team headed into '19. They were outscored by 34 runs, similar to the 77-win Mets and 78-win Twins. After everyone on earth said their 58-39 first-half record wasn't sustainable, they went 31-34 after the All-Star break.
FanGraphs projects Seattle to be the fourth-best team in the American League West next season, with a 79-83 record. You can quibble with that if you like, but it's clear the Mariners are looking up at the Astros and A's. They have arguably the weakest farm system in the game, and their big league roster isn't getting any younger.
Furthermore, the direction the Mariners should take really isn't clear. Due to the partitioning of the AL right now -- several elite teams, some non-competitive ones, and only a few in the middle -- a .500-ish team like Seattle might still have a shot at a Wild Card spot if things break right. Even if the Mariners wanted to blow it all up and start from scratch, they're not in great position to do that. They owe about $100 million in 2019 to the combination of Felix Hernandez, Dee Gordon, Kyle Seager, Robinson Cano and Mike Leake, all of whom are various levels of untradeable.
"We know what the Astros, Red Sox, A's, Yankees and Indians look like," Mariners general manager Jerry Dipoto said at the GM Meetings earlier this month. "We don't want to be a perpetual competitor for the second Wild Card. We want to build a championship roster. If that means in 2019 we field as competitive a team as we can while earmarking and gathering talent, we're not looking to rip our club down. We're just too talented to do that."
But we may have received a hint into the "soft rebuild" direction the Mariners might be taking in last week's Mike Zunino trade. Sure, Seattle sent its starting catcher to Tampa Bay, but they didn't do it for far-off prospects. While it was a five-player deal, the heart of it was trading two years of Zunino for four years of outfielder Mallex Smith, who is viewed as an upgrade from Guillermo Heredia (who went to Tampa Bay) and Gordon (as a center fielder).

That's probably the basic strategy, trying to trade shorter-term Major League talent for controllable young players or ready-now (or soon) prospects, in an attempt to rebuild the talent base while also keeping a competitive product on the field. It's likely what we'll see if the long-rumored James Paxton trade comes to fruition. While it seems counterintuitive to subtract one of baseball's best starting pitchers, the lefty is also an oft-injured talent who has two years left before free agency and may never be more valuable.
So in the same spirit, allow us to offer a similar proposal: Should the Mariners consider trading closer Edwin Diaz, who just had one of the best seasons ever? If you don't think it's a possibility, you haven't met Dipoto, who also told the Seattle Times, "I don't think there's ever a player that's off limits."
It may not be likely, but here's the argument:
Diaz really was that good
To say that Diaz had one of the greatest seasons for a reliever is in no way hyperbole. He just struck out 44.3 percent of the hitters he faced, which is the seventh-highest figure all-time. That has more than a little to do with the current state of baseball -- it wasn't even the highest figure of 2018, because Josh Hader had a 46.7 percent mark -- but you get the idea. Almost no one has had a year in which they missed bats like this.
Diaz added considerable strikeouts from 2017, when he whiffed 32 percent of hitters. He cut his walk rate nearly in half, from 11.5 percent to 6.1 percent. And Diaz did all that with just two pitches, a four-seam fastball (63 percent) and a slider (37 percent), which is why Seattle converted him to relief in the Minors in 2016.
To get the full picture of a pitcher's performance outside of defense and ballpark, we like to look at a Statcast™ metric called Expected wOBA, which accounts for quality of contact as well as strikeouts and walks. By that view, you can make a solid argument that Diaz was one of the three best pitchers in baseball.
Lowest Expected wOBA in 2018
.202 -- Sean Doolittle, Nationals
.208 -- Jose Leclerc, Rangers
.209 -- Edwin Diaz, Mariners
Now, Diaz turns only 25 in March, and he's doing this with elite velocity: His 97.3-mph four-seamer was tied for 19th of the 306 pitchers who threw at least 100 fastballs, and his 89.1-mph slider was ninth of the 197 who threw at least 100 sliders. If all of this sounds like a reason that the idea of trading Diaz sounds crazy, that's perfectly understandable. 

But it's also an argument that his value will never be higher. It's just about impossible to expect Diaz to give more than he gave in 2018, and even with that performance, the Mariners still finished eight games back in the AL Wild Card race. Now they head into '19 likely without free-agent DH Nelson Cruz and maybe without Paxton. What they have isn't enough. This is a way to spread talent around to more than a single relief-pitcher spot -- to say nothing of avoiding the risk of injury or underperformance from Diaz, which would hardly be unexpected considering the volatility of bullpen roles.
You don't want to trade Diaz, and the Mariners probably won't. But you realize how incredibly valued relievers are right now, and you look at the free-agent list and realize Diaz would immediately be the most valuable reliever available -- yes, above Craig Kimbrel -- and you think about the contenders that would absolutely love to add a pitcher of this caliber and you realize Seattle would have to at least listen. The players Diaz could bring in return could possibly help the 2019-21 Mariners more than the 170 or so innings he might be available to pitch himself.
There's some evidence that the Mariners should wait, that in-season trades would bring back a higher return. Let's save that for July. For right now, think about the teams they could talk to.
Finding the best return for Diaz
Here's a full and complete list of contending teams that think they have enough relief pitching: The Astros. Maybe, and they'd certainly find room for another elite arm. That's it. Every other contender is no doubt looking to upgrade the back end of its bullpen this winter.
That means the Mariners could basically name their price if they moved Diaz. They'd look at the Kimbrel trade in the 2015-16 offseason, when he had three seasons of control left and returned four prospects -- two on the Top 100 -- to San Diego. That would be the floor of their asking price if Seattle negotiated with these five most likely suitors:
1. Cardinals
You might have noticed just how inconsistent St. Louis' bullpen was in 2018, finishing 12th in the National League in ERA, which is why adding relief help is a top priority for the Cards this offseason.
"I think we understand the volatility of the relief market probably as well as anybody," president of baseball operations John Mozeliak said this week, clearly referring to the struggles of imports Greg Holland, Brett Cecil and Luke Gregerson.
There's a need in St. Louis, and there's potentially a fit, too. Ready-now catching prospect Carson Kelly is forever blocked by Yadier Molina, and the post-Zunino Mariners have no catcher. The Cardinals graduated a few strong young pitchers to the Majors in 2018 -- think Dakota Hudson and Austin Gomber. They even have a Cruz replacement in Jose Martinez, who isn't young (30) but won't be a free agent for four years and has hit .306/.369/.478 over the past two seasons. 

2. Dodgers
The Dodgers' inconsistent bullpen could certainly find use for Diaz, and they have their own blocked prospect in outfielder Alex Verdugo (MLB Pipeline.com's No. 1 Dodgers prospect), who has nowhere to play in Los Angeles and would be an upgrade of Ben Gamel in Seattle.
The Dodgers also have catching prospect Will Smith, who should be ready during 2019, and it's not hard to see a trade for Diaz including pitching prospect Dustin May or shortstop prospect Gavin Lux, both of whom could possibly be ready in '20. For more immediate impact, perhaps the Dodgers could offer lefty Caleb Ferguson, who broke out as an impact reliever in the Majors at 21 last season.
3. Cubs
Another NL contender, another unsettled bullpen.
"We're probably more focused on good relievers, honestly," Cubs GM Jed Hoyer said earlier this month. "The key is not just to have a good bullpen all year, but you've got to have that bullpen pitching well down the stretch, and part of it is having the depth to not overuse guys."
Maybe this is finally how Kyle Schwarber, who has long seemed destined for a DH role in the AL, moves? It would take more than that, obviously, perhaps something like pitcher Adbert Alzolay (the Cubs' No. 2 prospect, who should be Major League-ready in 2019), or maybe the Mariners like outfielder/second baseman Ian Happ's power, or maybe they prefer the long-term bet on No. 1 Cubs prospect Miguel Amaya, a catcher.
4. Braves
Atlanta may focus on a corner outfielder and a catcher this offseason, but the Braves would certainly take the opportunity to upgrade their bullpen, too. They certainly have plenty of young pitching to offer in a trade for Diaz, and might benefit by consolidating a few of those spots into a big bullpen upgrade.
We can't know who Seattle likes, but you can imagine the names: Bryse Wilson, Kolby Allard, Mike Soroka, Luiz Gohara (once a Mariners prospect), Kyle Wright and Touki Toussaint were all 22-or-under big leaguers in 2018. One could also Imagine Double-A outfielder Christian Pache, who may very well be the best outfield defender in the Minors, covering center field in Seattle.
5. Rockies
Colorado's focus is probably on adding bats, but the Rockies have needs in the bullpen, especially if Adam Ottavino leaves as a free agent. After last year's relatively unsuccessful bullpen shopping spree -- they spent $106 million on Bryan Shaw, Wade Davis and Jake McGee, who combined for a 5.41 ERA -- they may be hesitant to dip back into those waters again. So what do they do to keep up with the Dodgers? A trade for an elite reliever makes sense.
The Rockies have pieces to send back, too, starting with top prospect Brendan Rodgers, an infielder who can play shortstop or second. (In this scenario, the Mariners might trade Jean Segura or Gordon; Colorado can still slot Garrett Hampson or Ryan McMahon at second.) Continue with a prospect like ready-now Peter Lambert, or if Seattle prefers a big leaguer like Tyler Anderson or Antonio Senzatela, well, that's on the table too.
Many more
This is not a complete list. The Yankees would be interested, though they'd likely prefer Paxton. The Red Sox would be, too, though they may not have the prospects. The Phillies could probably build something around young pitchers and outfielders. Maybe the Brewers just want to build the best bullpen in the history of the sport.