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.