That's a long way from where he was early in the year. Diaz, acquired from the Angels last December in a trade for infielder Josh Rutledge, has a high-90s fastball and a swing-and-miss slider, but he struggled with control.
Through his first 25 games with the Isotopes, Diaz had 21 walks, 19 strikeouts and a 6.08 ERA. But over his final 22 games, he recorded a 3.18 ERA and a 1.9 strikeout-to-walk ratio.
With the Rockies, Diaz has faced 27 batters, surrendering just two walks and five hits.
"I think the hitters were getting a good look at the ball early on, and he tweaked some things mechanically to try to hide the ball a little longer," Rockies manager Walt Weiss said. "A guy throwing that hard with a slider that hard, you shouldn't get hit hard -- and he was early in the season. So I think a lot of it was just due to the fact that hitters were seeing the ball a long time.
"He was a guy that kind of spun off, so they tried to get his direction better going to home plate and not spinning off. When you spin off, you open up sooner, and therefore, hitters see the ball longer. So I think it's all tied together."
Although Diaz had pitched in only five Major League games when he joined Colorado, it hasn't shown.
"He seems pretty composed to when he's out there," Weiss said. "Doesn't seem to get rattled by the big spots."
Dargan Southard is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.