Arroyo, who threw a three-hit complete-game shutout in his first-half finale on July 6 at San Diego, gave up five runs on eight hits, and had already allowed four runs and five hits through his first five batters.
The game started amid ominous news, when it was learned that team leader Joey Votto could miss up to a month because he needs arthroscopic surgery on his left knee. It didn't get any better for Cincinnati once it started.
After three straight singles loaded the bases for Arizona in the first inning, the big blow was Miguel Montero's three-run double to the left-field corner. Justin Upton followed with an RBI single to make it 4-0.
"They didn't hit Bronson real hard that first inning, but they found the holes," Baker said. "We're trying to figure out how to piece together seven innings without tearing up my whole bullpen, especially with Johnny [Cueto] coming back tomorrow from being out with a blister."
The Reds notched one run in the bottom of the first inning against D-backs rookie lefty Wade Miley. Todd Frazier, filling in for Votto, hit a two-out ground-rule to right-center field and scored on a Brandon Phillips RBI double to center field.
Two more Cincinnati runs scored in the second inning as Drew Stubbs' two-out, two-run single to left field scored Chris Heisey and Ryan Hanigan.
But leading off the Arizona third inning, Montero hit a full-count pitch from Arroyo into the right-field seats to make it a two-run game.
Arroyo, who had velocity issues last season as he battled through mononucleosis and back soreness, wasn't worried that he didn't feel strong this time.
"You can only come out of the bullpen with what you've got," Arroyo said. "You have to try and find a way to win. If it happens three times in a row, you'd be concerned."
Alfredo Simon, who got his first victory with Cincinnati against St. Louis on Sunday, returned Monday with three scoreless innings of relief to keep his team in the game. Bill Bray survived a three-walk seventh inning without allowing a run, and Jose Arredondo and Logan Ondrusek added a scoreless inning each.
"Their bullpen did the job against us and our bullpen did a good job against them to hold the score to where it was," Baker said. "It wasn't a very exciting night."
The D-backs came to town losers of three straight games after being swept by the Cubs while scoring all of three runs in the series.
"We should have won today's game, regardless of who is playing or not," Phillips said. "You've got to win games. It stinks that we lost today. They're a great team, but when they're down, you have to beat those guys. It was a tough loss for us today."