The 22-year-old has been imposing this year with 66 strikeouts and 13 walks in 36 innings, but his command was shaky his previous two outings as he needed 27 and 34 pitches to maneuver through two difficult saves last week.
Manager Scott Servais gave the youngster two days off so he could reboot, even though it came at a cost Sunday when the Brewers bounced back for four runs in the ninth against his replacements to pull out a 7-6 win. But the time off appeared to pay off against the Yankees after Diaz recovered from a four-pitch leadoff walk to Brian McCann.
Chase Headley bounced a one-out single up the middle to put two on, and Diaz heightened the suspense by committing a balk with Mark Teixeira pinch-hitting to put both runners in scoring position, but Diaz got Teixeira to pop out to left and then induced a game-ending grounder to second by Brett Gardner.
"I felt better today," Diaz said. "My fastball command was a little better. I worked a lot the past two days and I felt the difference. It got a little bit crazy when I started [with the walk], but then I relaxed and made my pitches and it was good."
And the Mariners breathed a huge sigh of relief when he escaped with both the win and his confidence still intact.
"He's flying open and the ball leaks up the arm side," said Servais. "He's got to learn how to make those adjustments. When he goes to the slider, he gets his release point back in there and stays gathered and closed and the ball goes where it's supposed to. But the fastball right now is a little bit of a challenge.
"But he regrouped, got back into it. It's a process, and that's what happens when you're 22 years old."
Said catcher Mike Zunino: "You know he's got the stuff to get out of that situation. His big thing is he's just got to trust it. He doesn't need to be too fine. He has to pound the strike zone, and he got back in that mentality and showed what he can really do to win that ballgame."
And in the end, the youngster from Puerto Rico said it was perfect enough.
"I got 10 saves to start my career," he said with a smile. "I want to save as many games as I can to help this team make the playoffs."
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.