The defeat dropped Cincinnati to 93-62 and pushed the Reds one game behind the Nationals -- who defeated the Phillies, 8-4 -- in the race for the National League's best record and the No. 1 seed in the playoffs.
The loss also dropped Arroyo to 12-9, but the 13-year veteran said he feels he's pitched better than his record shows.
"I think every starting pitcher feels that way most of the time, especially this club this year, because we've run a lot of quality starts out there," said Arroyo, who has chalked up 31 starts this season. "I definitely feel like I could have won three or four more ballgames, especially here at home. I've thrown probably four gems this year that wound up getting a no-decision or a loss, but you look back on most years and that can happen."
His 3.70 ERA is the lowest he has posted since his first year in Cincinnati back in 2006 (3.29). After the Reds readjusted the starting rotation last week, Arroyo will likely enter the postseason as the No. 2 starter behind Johnny Cueto.
Arroyo entered Wednesday night's contest on a tear, picking up five wins in his past eight starts and posting a 2.60 ERA with a 1.03 WHIP during that span.
Working with an early one-run cushion, thanks to Joey Votto's RBI double off Brewers starter Shaun Marcum (6-4) in the first inning, the crafty 35-year-old was sailing smoothly, retiring eight of the first nine batters he faced before a two-out rally in the third inning led to three Brewers runs.
"It does feel like whenever we face [Arroyo], he does do well against us," said Norichika Aoki through his translator, Kosuke Inaji. "Even though they scored first, we just felt we had to get one back as quickly as possible and go on a run from there."
Aoki, who went 3-for-4, launched a solo home run to begin a parade of five consecutive two-out hits in the frame, giving the Brewers a 3-1 lead. Milwaukee tacked on another run in the sixth off Arroyo, who gave up four runs on eight hits over six innings against one of the hottest-hitting teams in baseball.
But he wasn't alone. The Reds' bullpen -- consisting of Logan Ondrusek, Sam LeCure and Alfredo Simon -- couldn't stop the bleeding after Arroyo's departure, giving up runs in the seventh, eighth and ninth innings, with all four pitchers combining to give up a season-high nine extra-base hits.
"We just couldn't get anything going offensively, and they could," said bench coach and acting manager Chris Speier. "This team came in swinging the bats really well. They got the clutch hits when they needed to."
The three Reds relievers surrendered a combined four runs on five hits in three innings, including two home runs -- to Ryan Braun, his 41st, and Jonathan Lucroy. All three righties are also fighting for their spot on the 25-man playoff roster the Reds plan to set when the club gets to St. Louis next week.
The Reds know they will need their offense to improve if they want to make a deep run in the postseason. Cincinnati has scored the second fewest runs in the NL in September (70), beating out only the Dodgers (65).
The struggles certainly haven't gone unnoticed.
"To tell you the truth, I don't know what it is," said Brandon Phillips, who went 0-for-5 Wednesday. "I can just speak for myself, I don't know why I'm not getting any hits. I don't know why anybody else isn't getting their hits. Maybe because we know we clinched and we're going to the playoffs, maybe because we think we can turn the switch on or something like that. I don't know."