CINCINNATI -- Reds ace Luis Castillo more than carried his weight from the mound vs. the D-backs on Saturday. But Castillo couldn’t do it all to carry his team for a win -- not when Arizona rookie left-hander Alex Young was pitching marvelously, not when the Reds’ offense was flat and not when mistakes were made.

Castillo worked a season-high-tying 7 2/3 innings and struck out 10, but was on the short end of a 2-0 loss at Great American Ball Park. He allowed both runs and two of his three hits in the decisive top of the fourth inning.

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Young held the Reds to two hits over eight innings -- both of which were infield singles -- and struck out 12 while walking one batter.

“It’s one of the best games I’ve pitched this year,” Castillo said via translator Julio Morillo. “In the fourth inning, I made a couple of mistakes that cost me two runs and cost me the game.”

Jarrod Dyson opened the fourth inning with a walk against Castillo and stole second base. Ketel Marte grounded a single through the middle for Arizona’s first hit of the day and the game’s first run as Dyson came around to score. 

“I was trying to throw my two-seamer to one spot and I totally missed the spot,” Castillo said in taking some blame.

Eduardo Escobar was next with a fly ball to left field, where Josh VanMeter took a poor route and watched as the ball landed safely for a double that put two runners in scoring position. Christian Walker grounded a ball off Castillo’s leg and although the out was recorded, it still allowed Marte to score.

“It was a tough inning. That inning could have gone completely different,” Reds manager David Bell said. “Not to take anything away from the Diamondbacks. I could have seen him getting a complete game today and getting a win. It didn’t happen.”

Castillo’s record dropped to 14-6 with a 3.21 ERA in 29 starts, while he set a new career high with 173 2/3 innings pitched. For at least one start, he missed out on being Cincinnati’s first 15-game winner since Johnny Cueto (20 victories) and Alfredo Simon (15) both reached the mark in 2014.

On the plus side, Castillo crossed off 200 strikeouts for the season. He’s the 12th different Reds pitcher to achieve that mark, and the first since Cueto in ‘14. As has often been the case during his successful starts, Castillo had his sharp changeup and he used it 40 times. According to Statcast, he notched eight of his strikeouts with it -- six on swings and two called. His four-seam fastball picked up the other two K’s.

“He’s been so good, so consistent all year. He maybe wasn’t quite as good recently, that’s hard to really say that,” Bell said. “It looked like any point this year where he was at the top of his game. He was that good today.”

A first-time All-Star this season, Castillo was 8-3 with a 2.29 ERA in 18 first-half starts. Over 11 starts in the second half, he is 6-3 with a 4.66 ERA.

“Overall, I feel really happy about my second half,” Castillo said. “This game, sometimes you’re up and sometimes you’re down. But the main thing you have to do is come to the park, improve and continue to get better.”

It was the sixth time this season that Castillo received less than three runs of support. He is 1-4 with a 2.45 ERA in those games. Young had a big part to do with the lack of run support Saturday.

While retiring 12 of 13 batters, Young held the Reds hitless for the first four innings with Jose Iglesias beating out a ground ball to the shortstop for the club’s first hit to leadoff the fifth. Iglesias bolted for second base as Freddy Galvis followed with a line drive to right field. As the ball was about to be caught by Dyson, Iglesias appeared to freeze and watched the play develop. Dyson caught the ball and threw to first base for a double play.

“It wasn’t a hit and run, he was straight stealing. I don’t see any way he could’ve gotten back,” Bell said. “The other thing is, you don’t know if that ball is going to get caught. Once you get into no-man’s land, it’s just one of those plays. There wasn’t a mistake there. It was more unfortunate timing.”

Young, who was not on the 40-man roster and was a reliever at Triple-A when the season started, retired the next 10 batters in a row. That ended when Curt Casali notched a two-out infield single in the eighth when his grounder skipped off the third-base bag.

With one out in the bottom of the ninth and Alex Blandino on after a pinch-hit single, Joey Votto drew a four-pitch walk against lefty Andrew Chafin. Righty Jimmie Sherfy struck out Eugenio Suarez on three straight sliders and then struck out Aristides Aquino to end the game.