"He's not an easy guy to face," Arizona manager Chip Hale said.
Indeed, the 13 strikeouts Sunday were the most by a Reds rookie since Gary Nolan fanned 15 on June 7, 1967, against San Francisco.
"I tried to keep the same pattern I had back in Arizona," said Iglesias (3-5, 3.93 ERA), who allowed one run in six innings of a 2-0 Aug. 7 loss against the D-backs.
"I was trying to attack them in the beginning. I was trying to get ahead of the hitters using my slider. My slider was working really well today as well as my changeup. My changeup has been working better the last few outings. I also mixed in the curveball."
Iglesias, speaking through interpreter Tomas Veras, said the most disappointing moment of the day came in the sixth inning. His throwing error helped Ender Inciarte score from second to extend a 1-0 Arizona lead to 2-0.
Reds first baseman Joey Votto's errant throw to Iglesias, covering first base, allowed Paul Goldschmidt to reach safely. Inciarte broke for home and scored when the throw from Iglesias sailed over the head, and past the glove, of catcher Brayan Pena.
"I got angry," Iglesias said. "If you look at the play, I had plenty of time to make a good throw and didn't realize I had the chance to do it. I got angry with myself because I could have made a better throw in that situation."
Still, Iglesias set a career high for strikeouts and has allowed only one earned run over his last 19 innings -- Welington Castillo's fourth-inning solo homer on Sunday.
Those who have observed the 25-year-old Cuban-born pitcher for some time say he has a unique ability to adjust to hitters based on what he observes during each at-bat.
"I always have paid attention to that," Iglesias said. "After you go through the batting order once and you see how they're going to position themselves in the batter's box and how they swing and make contact with the pitches you throw to them, you start making judgments based on that. I actually am continually looking at how they swing and how they stand to make my game plan."
Those game plans appear to be working for the young pitcher who has allowed more than three earned runs just three times in 12 starts.
"He did a terrific job and he spared us beating up our bullpen again," Reds manager Bryan Price said.