Diaz struck out three more in 1 1/3 innings in Sunday's 7-6 loss to the Cubs, including mowing down Addison Russell on three pitches to strand Ben Zobrist at third after entering the game with two out in the seventh.
That strikeout ability is a weapon for Mariners manager Scott Servais, who has gradually moved Diaz into the team's eighth-inning setup role as he's gained experience.
"It doesn't get any bigger," Servais said of the punchout potential. "Hitters know it. Now it's got to the point he's actually got some track record. So they look at the numbers and the strikeouts and, 'What does this guy have?' Here it is. You get their attention. But still, being able to do it is about executing. He's got great stuff -- a lively fastball and a really good slider."
Diaz consistently throws his fastball in the upper 90s and touches 100 mph at times, while the development of his slider in combination with that heat has been critical. And the youngster has some presence as well.
"His demeanor, I think from the first day he got here, he felt like he belonged," Servais said. "Credit to our veteran group to make him feel like he was a part of it right away. I think once they saw him throw the first time, it makes it easier for guys to put their arm around him and welcome him in. That always helps."
Servais thought Diaz took a big step in Saturday's 4-1 win over the Cubs when he entered with a two-run lead in the eighth and gave up a single and one-out walk, but then punched out pinch hitter Anthony Rizzo and leadoff man Dexter Fowler.
"The thing that's been most impressive to me is that he has not backed down," Servais said. "I thought yesterday was a great test for him in this environment, against this team, and not off to a great start because he was overthrowing a little bit to the first couple hitters. But he made the adjustment and got through it. That was really big for him."
Diaz said he indeed learned from the situation in a tense game in front of 40,000 fans at Wrigley Field against the team with the best record in the Majors.
"When I first started pitching, I was too excited," said the youngster from Puerto Rico. "Then when I had the walk, the pitching coach [Mel Stottlemyre Jr.] came out and talked to me and told me, 'Hey, do what you know how to do. Just relax and pitch.' And I did that. I slowed down and just did my job."
Diaz converted to a relief role on May 10 while in Double-A Jackson and his on-the-job training is still underway.
"I like to pitch in that kind of situation," he said. "I like to feel pressure when I pitch. That makes me work hard and do my best and I have fun doing that.
"That game and situation helped the team to win. If I put down a zero and [closer Steve] Cishek does, too, we win. I felt part of the win yesterday. We did our job."
Diaz has put up a 1.80 ERA and the strikeouts-per-nine innings rate is well ahead of the next-closest pitchers in MLB, Dellin Betances (15.91) and Andrew Miller (15.29), now with the Indians. Chapman is 10th on the list at 12.97.