On Tuesday night, he put those feelings to good use. Diaz locked down his first Major League save by striking out the side in the ninth to preserve the Mariners' 5-4 win over the Red Sox.
"We all know how good he is," said veteran second baseman Robinson Cano, who hit a go-ahead, three-run homer in the eighth. "First time being a closer, it's tough for everyone his first time, especially at home. And he was the same guy. Lights out."
Diaz faced four batters in the inning, issuing one walk.
Servais had told Diaz about his new role after Mariners closer Steve Cishek gave up the winning home run in the Mariners' 2-1 loss to the Red Sox on Monday. The night before, Cishek also blew a save in Chicago by allowing a game-tying three runs in the ninth inning en route to a Mariners' 7-6, 12-inning loss.
Even before Monday's game, Servais had talked to Cishek about moving away from having a "lockdown closer," in order to alleviate some of the pressure on him and get him back on track. Monday night only reinforced that thought.
"He understands," Servais said. "Obviously the results of last night's game, not good. He's got to get back in a good spot mentally, physically. The ball's just not coming out of his hand well. And he knows it."
Diaz has not allowed a run in his past 10 outings, dating back to July 8. He has 13 holds this season, including four in his past four appearances.
"He hasn't backed off," Servais said. "He's been very aggressive. He believes he belongs in this league. He certainly has produced at a level that he should believe that he belongs in this league. It's hard to do what he's done to this point."
The move isn't necessarily permanent. Diaz is only 22, and Servais doesn't want to push him beyond his limits. Diaz has not played a full Major League season, after all. Other pitchers may come in to get save chances depending on matchups and Diaz's workload.
"When we signed Steve Cishek, we signed him to be our closer," Servais said. "We put him in that role. For the most part he's done a pretty good job. He has blown a few saves. The last couple nights have been rough. But he was signed to be the closer. Edwin Diaz is a pitcher in our bullpen that's been very good, and if it lines up, he'll pitch the ninth inning."
Fantasy spin | Fred Zinkie (@FredZinkieMLB) After opening the year with a strong two months, Cishek has posted a 6.55 ERA since the start of July, losing the trust of manager Scott Servais and fantasy owners alike. Those in need of saves should rush out to add the rookie Diaz, who has a 1.80 ERA and a ridiculous 17.6 K/9 rate across 25 innings. If Diaz is unavailable, owners could consider new saves sources in Milwaukee (Tyler Thornburg) and Arizona (Jake Barrett).
Maddie Lee is a reporter for MLB.com based in Seattle.