Diaz ended 2018 as the top closer and looks to be one of the first off the board in 2019 after an offseason trade to the Mets. Here's the thing: amassing another 61 save chances with 57 conversions is a long shot. Last year, Diaz tied for the second-most saves in history. Even with comparable skills, 15-20 fewer is a fair expectation. Speaking of skills, they're elite. His 18.9 SwStr% was bested by only Josh Hader, and just barely at that. His K-BB% was tops in the league. Diaz throws a 98-mph fastball almost two-thirds of the time, complemented by a 90-mph slider. There's really nothing to point to in terms of luck as Diaz's BABIP, HR/FB and LOB% were all within expected ranges. The skills, hence strikeouts and ratios, are real and repeatable and thus worthy of chasing aggressively. Just don't count on another 50-save season.

The road was a bit rocky for Diaz in 2017, with the Mariners temporarily relieving him of closer duties on a couple occasions, but when it was all said and done Diaz tied for fourth in the American League in saves. He converted 21 of 23 save opportunities in the second half and struck out 32 percent of the batters he faced overall using a combination of high-90s fastballs and high-80s sliders. Diaz can be his own worst enemy with the walks, and the uptick in home runs last season is a concern, although he was better in that regard down the stretch (two homers allowed in his final 30.1 innings). His stuff is electric most of the time and there aren't any proven options behind him in the Seattle bullpen, so Diaz should be given every chance to remain in the closer role throughout 2018. However, he's not exactly a "safe" investment given the slip in performance from 2016 and the general year-to-year unpredictability of major-league relievers.

Seattle's save chances were dominated in the first half by Steve Cishek. However, after Cishek landed on the disabled list, Diaz, once a top starting pitching prospect, took the reins and never looked back. He dazzled in the bullpen over the final two-plus months, going 18-for-21 in save chances. Diaz ranked sixth in second-half K/9 (14.3) among arms with 20-plus appearances, behind Ken Giles, Kenley Jansen, Dellin Betances, Craig Kimbrel and Aroldis Chapman. On the season, he finished fourth among relievers with an 18.5 swinging-strike percentage. While his his walk rate was a sparkling 2.6 BB/9, he has bouts of inefficiency and must aim to get ahead in more at-bats. Still, the right-hander, who turns 23 in March, boasts outstanding peripherals (33.6 K-BB%) and electric stuff. Even with some correction perhaps coming, Diaz could deliver numbers befitting a top-five fantasy closer at a discount relative to bigger names.

It is a sad state of affairs when a pitcher who finished the previous season with a 4.57 ERA in 104.1 innings at Double-A has a claim as the best prospect in a farm system, but that is where the Mariners are at. Diaz is not nearly as bad as that ERA suggests — he had a 1.70 ERA in 37 innings at High-A Bakersfield before his promotion and his 3.22 FIP at Double-A Jackson suggests he was pretty unlucky in Southern League play. His 145:46 K:BB in 141.1 innings illustrates his potential to be an average or better bat misser with an average command profile in the big leagues, essentially serving as a true No. 3 starter who should benefit from playing half his games in Safeco Field. Diaz should spend most, if not all of his age-22 season at Triple-A Tacoma, potentially getting some late-season spot starts with the Mariners, but more likely figuring into the rotation plans in 2017.

A third-round pick out of Puerto Rico in the 2012 draft, Diaz held his own at Low-A Clinton last year as a 20-year-old after dominating in rookie ball in 2013. He has a low-to-mid-90s fastball, a good slider, and a developing changeup. His control needs some work, but he has plenty of time to hone those skills. He will also continue to fill out. His 6-foot-2, 178-pound frame is about 20 pounds more than his draft weight, but he's bound to get bigger and stronger still. The organization's co-starting pitcher of the year in 2014 (along with Jordan Pries), Diaz is a ways away, but he has plenty of upside and is worth keeping an eye on.