Diaz can top 100 mph with his fastball, but it has been his slider that has been the difference in recent outings. He has 29 strikeouts in 151/3 innings since being called up from Class AA Jackson.

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HOUSTON — His fastball draws oohs and gasps from velocity-starved fans, who are awed by radar guns flashing, 98, 99, 100 and even 101 mph. Reliever Edwin Diaz can certainly throw a baseball hard. No pitcher in the Mariners bullpen or upper levels of the organization can match the speed of his hissing fastball.

Since being called up from Class AA Jackson to make his major-league debut, he’s been brilliant, appearing in 14 games and posting a 1.76 ERA that includes 29 strikeouts in 151/3 innings and 69 hitters faced.

He’s become one of Seattle’s best relievers and gives the Mariners a power arm that has been missing since Tony Zych went on the disabled list with rotator cuff tendinitis.

He’s been most effective in his recent outings. His last 10 outs have all come via strikeout. He’s fanned 12 of the last 20 hitters he’s faced.

But it’s not the blazing fastball that’s the reason for the success. Manager Scott Servais credits Diaz’s slider as the key for the recent run of punchouts.

“The development of his slider has really been the difference,” Servais said. “Everybody knows he throws hard, we knew that, we knew were getting that. But the development of the secondary pitch has been huge.”

Of the 57 sliders Diaz has thrown this year, he’s generated swings and misses on 35 percent of them, per Brooks Baseball.

That development has occurred at the big-league level with a little help from veteran Joaquin Benoit.

“There’s a marked difference from when he first got here to where it is now,” Servais said. “He’s changed the grip a little bit. He’s changed his thought process on how and when he’s throwing it. That’s great. Players need to continued to develop when they get to the big leagues, the young guys especially, when they don’t have a lot of minor-league track record. It’s nice to see it evolve. He knows it and feels good about it.”

Benoit showed a slightly adjusted grip about 10 days ago. It gave Diaz’s slider sharper late-breaking movement with more depth.

“They’re missing it,” Servais said. “It’s hard. It’s late. When you throw that hard at 97-98 mph, guys have to cheat for the fastball. And when the breaking ball is coming out of the same window with the late depth at 87-88, it’s really good. He had one he left in the middle of the plate to Trumbo. If he throws that in a good spot, Trumbo probably doesn’t hit it. That’s what separates the average breaking pitch to the above-average breaking pitch.”

Notes

Felix Hernandez will throw a 45-pitch simulated game on Wednesday afternoon. It will be three 15-pitch innings. If that goes well, the plan would be for him to pitch on Sunday in Everett and then five days later in Tacoma.

Seth Smith’s streak of consecutive games with a home run ended at four. Smith went 0 for 4 with three strikeouts in the loss.

• The Mariners are carrying an extra reliever on the 25-man roster, but it’s unlikely they will have an eight-man bullpen after the all-star break. David Rollins , the extra left-hander in the bullpen, will probably be sent back to Class AAA Tacoma

“We’ll see,” Servais said. “I think we are getting to the point where you are kind of looking ahead to the break and you reshuffle. But it could be before that. Once you get the rotation stabilized, now that extra guy down there, you don’t really use that guy as much.”