Arroyo (8-7) allowed three hits, including Brian McCann's two-out solo homer in the seventh, and walked one.
"I had good stuff today for the first time in a little while," Arroyo said. "I had a little bit of zip on my fastball and a little sharper breaking ball, which I haven't had in a while."
Arroyo had dropped two of his last three starts. But he was in control against the Braves and didn't allow a baserunner to reach second before McCann's homer.
"I commanded the game the way I wanted to and the mistakes I did make they popped the balls to left field," Arroyo said. "The ball rolled my way a good bit today. It just seemed like nothing was going wrong."
Atlanta added two more names to its list of injured outfielders, with brothers B.J. and Justin Upton unable to finish the loss. Jason Heyward was held out one day after he strained his right hamstring.
"There must be something in the water," said B.J. Upton, who strained his right adductor muscle in the first inning.
Justin Upton strained his left calf while running to first base in the seventh. The Braves' patchwork outfield at the end of the game included Tyler Pastornicky, Reed Johnson and rookie Joey Terdoslavich.
The team announced after the loss that Jose Constanza will be recalled from Triple-A Gwinnett before Saturday's game. A corresponding move was not announced.
Pastornicky walked with two outs in the ninth and went to second on defensive indifference. He scored on Freddie Freeman's single, but McCann popped out to end the game.
The Reds jumped on Kris Medlen (6-9) right at the start. Shin-Soo Choo singled and moved to second on Derrick Robinson's bunt single. Medlen walked Joey Votto to set up Phillips' two-run single to center. Votto scored on Todd Frazier's triple off B.J. Upton's glove and left knee.
Upton's right knee dug into the grass as he attempted to make the catch. He appeared to favor his right side as he walked slowly off the field, accompanied by assistant trainer Jim Lovell.
Upton said he "kind of dove" for Frazier's line drive.
"With the backdrop, it's tough to see sometimes," he said, before adding the injury is "not too bad."
"We'll re-evaluate tomorrow, see how it feels," he said.
Justin Upton said his calf "kind of locked up" as he was running to first base.
It was another rough night for Atlanta's outfield, forcing the Braves to consider their options with the All-Star break coming up.
"We're talking through it right now," manager Fredi Gonzalez said. "We've got three guys banged up."
The list of injured outfielders actually is longer than the three starters.
General manager Frank Wren announced during the game that Jordan Schafer's right ankle injury is more serious than originally believed. Schafer went on the 15-day disabled list on July 4 with what was called an ankle contusion, but a MRI on Friday revealed a stress fracture. Wren said the normal recovery period for the injury is six weeks "and he's two to three weeks into it."
Evan Gattis, who has made 11 starts in left field, could return from an oblique injury after the All-Star break.
The Reds chased Medlen in the fifth. Robinson singled and moved to third on Votto's double before Medlen hit Phillips with a pitch to load the bases. Left-hander Alex Wood then came in and gave up a sacrifice fly to Jay Bruce.
Medlen was charged with four runs and nine hits. Medlen, who was 4-1 with a 2.14 ERA in five June starts, has allowed a combined 10 runs in two straight losses.
"I look at my mechanics after every game and I'm doing the same thing," Medlen said. "I'm trying to make adjustments and it comes down to, in the game, making the adjustments stick. It hasn't gotten there for me."
The Braves had only two hits before McCann's seventh-inning homer. Medlen singled with two out in the third and Justin Upton singled to right field in the fourth. Upton was thrown out at second by Bruce.
- The run in the ninth was the first allowed by the Reds' bullpen since June 28 at Texas. The streaks of 10 games and 33 2-3 innings with allowing a run by the bullpen set team records.
- Reds LHP Sean Marshall threw to batters before the game. Reds manager Dusty Baker said Marshall is closer to returning from his second DL stint with a sore left shoulder.
- RHP Homer Bailey will make his second start since throwing his second career no-hitter against San Francisco on July 2 when he faces Atlanta's Mike Minor on Saturday. Bailey followed the no-hitter with a loss at Milwaukee on Monday.