The hard-throwing right-hander, called up from Double-A Jackson on Saturday, made his big league debut on Monday night with a perfect seventh inning, a bright spot in the Mariners' 3-1 loss to the Indians.
"Obviously he was excited to be here," manager Scott Servais said. "I think our people in player development, on the Minor League side, I need to tip my hat to them. Awesome job getting that young kid ready to pitch in the big leagues."
Diaz, who was 3-3 with a 2.21 ERA at Jackson, sandwiched a pair of 5-3 groundouts around a strikeout. The 22-year-old Diaz, the Mariners' third-round selection in the 2012 Draft, made his final 10 appearances for Jackson out of the bullpen after six starts.
"I think he's only pitched nine or 10 outings out of the bullpen in his career," Servais said. "He was calm, he knew what he was going to do and he executed. Great job. Certainly I think he's earned the chance to stay for a little longer."
Diaz said he tried to convince himself it was just another relief situation.
"When I was warming up, I was thinking it was the same thing here as Double-A," Diaz said. "I need to make pitches and get outs.
"I was more nervous in the bullpen than on the mound. I tried to keep it simple. After the first pitch, I said, 'Let's go,' and got my first inning."
According to MLB Statcast™, Diaz hit 101.2 mph on a pitch to Rajai Davis, the third batter he faced.
"That's the reports," Servais said. "He's got a really lively fastball. Controlling it through the course of his career, he's not walked many guys. He throws the ball over the plate. Nice to see the slider, too. His willingness to go to it, not afraid and located it pretty well."
In his Minor League career, Diaz struck out 409 batters with 130 walks in 386 1/3 innings. With Jackson this season, he struck out 54 while walking seven.
Mariners catcher Chris Iannetta also was impressed with Diaz's performance, but he cautioned that it was just one game.
"It's awesome to come up and make your debut, especially when you have success. It's always fun," Iannetta said. "We'll just keep working with him. It's a long road.
"Hopefully, from all the signs, it looks like he has a very bright future ahead of him. His stuff was great, location was even better."