Diaz, a 22-year-old native of Puerto Rico, is regarded as the Mariners' No. 2 overall prospect by MLBPipeline.com, and he opened the year going 2-3 with a 3.00 ERA in Jackson. He earned Minor League Pitcher of the Year honors for Seattle last season while splitting time between Clinton and Jackson.
But Dipoto and his new regime see Diaz's upside more as a bullpen candidate, and he made his first relief outing Tuesday with a perfect eighth inning for Jackson.
"We felt like he had the potential to be a Major League starter, but he has the potential to be an impact Major League reliever, and to me, one supersedes the other," Dipoto said.
The 6-foot-3, 165-pound Diaz has an excellent fastball and a good slider, but he's had trouble developing a quality changeup, and Dipoto sees a combination that seems better suited to the 'pen.
"As nice as his progression has been as a starter, we felt like his ceiling was probably more back of the rotation because of the lack of the third pitch and the general wonder about how he could handle the innings, because it's not the largest frame among the pitchers in our system," Dipoto said. "But as a reliever, if he can acclimate himself to the bounce back and show the resiliency to be a bullpen guy, does the stuff tick up?
"If [Tuesday] night, the first one, is any indication -- he threw a really clean inning, 13 pitches, nine strikes. He was up to 98 mph, sitting [at] 96 and looked every bit of what we were hoping it would look like. Now we have to see if it looks like that in outing No. 2."
The Mariners made the same move with right-hander Dan Altavilla in the spring. The 2014 fifth-round Draft pick is Seattle's No. 20-ranked prospect and had gone 6-12 with a 4.07 ERA at Class A Advanced Bakersfield in his first full season of pro ball, but he is now working as Jackson's closer.
Dipoto doesn't foresee any other conversions at the moment, and he likes the development of the club's pitching down below.
"We really like the progress Emilio Pagan is making. He's off to great start in Double-A [with a 0.60 ERA in 15 innings of relief]," Dipoto said. "Our bullpens in general have been pretty good in the Minor Leagues. Among the starting pitchers, Andrew Moore is off to a really good start [with a 1.48 ERA in seven starts at Bakersfield], and there's a good likelihood he'll see some shift in location before too long. He's performing his way out of that league.
"At the Triple-A level, we like the starting rotation that we broke with. James Paxton has been great. Adrian Sampson has been throwing the ball extremely well. I feel like we got Joe Wieland and Cody Martin back on track these last couple outings. They got a little wobbly there for a while, but the team has played really well. I couldn't be happier with the depth we've accrued."