But Diaz isn't the sole reason for the turnaround as the entire Mariners bullpen has been lights out since that switch was made and Seattle has taken advantage by roaring back into playoff contention.
In addition to Diaz, the Mariners have received a big boost from the acquisition of hard-throwing right-hander Arquimedes Caminero from the Pirates, the continued excellence of midseason addition Tom Wilhelmsen and the return to health of Nick Vincent.
It's added up to a group that has the lowest bullpen ERA in the Majors in August at 1.25, with 33 strikeouts and seven walks in 43 1/3 innings while holding opponents to a .185 batting average.
"It's key to the run we're on," manager Scott Servais said prior to Monday's series opener with the Angels. "Our offense has been there some nights and some nights it hasn't, but our bullpen has kept us in games. Teams haven't gotten too far out ahead of us and it's been key. It's been fun to watch. The guys have a lot of confidence down there and we've got to keep them going."
Caminero has allowed just three hits with no runs or walks while striking out five in his first six innings with Seattle. Wilhelmsen has a 1.65 ERA and has stranded 15 of 16 inherited runners in 18 games with Seattle after posting a 10.55 ERA in 21 games for Texas.
"We've added some pieces," Servais said. "Wilhelmsen has been throwing the ball great, obviously you add Caminero, we get Vincent back, [Drew] Storen has helped out. [Vidal] Nuno has been the solid, steady guy down there all year. It's a good group."
• The Mariners could add another piece to that 'pen by as soon as the next homestand as former closer Steve Cishek made his first Minor League rehab outing on Monday night at Everett since being sidelined on Aug. 2 by a slight tear in his hip labrum. Cishek allowed two hits and a run, but struck out three in his one inning against Salem-Keizer.
• Rookie right-hander Tony Zych made his second rehab outing on Monday since missing three months with right shoulder tendinitis, giving up one walk and striking out two in a scoreless inning for Double-A Jackson. Servais said there's no timetable on Zych's return, given his long absence.
• Evan Scribner, who has been on the DL the entire season with a strained right lat muscle, also threw a scoreless inning with one walk and one strikeout for Jackson in his fourth rehab outing. The 31-year-old was acquired from the A's last winter, but was hurt early in Spring Training.
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.