"I just know that some teams have to lose some ballgames and all we can worry about is trying to win every game," Arroyo said.
Rich Aurilia's RBI single and Ross' shot off Carlos Marmol (5-7) during a four-run fourth were enough to give the Reds their sixth victory in 19 games.
Arroyo (14-9) won his fourth straight start, allowing four hits while striking out seven and walking three. It was the second scoreless outing in three starts for Arroyo, who threw the first shutout of his career against San Francisco on Sept 5.
Scott Schoeneweis pitched the ninth to finish the combined five-hitter.
It was small consolation for a Cincinnati team that saw its playoff hopes all but evaporate this week by losing two of three to wild-card leader San Diego. The Reds closed within 3½ games of the Padres.
It's been a long fall from where Cincinnati was after beating San Francisco on Aug. 24: less than a percentage point behind St. Louis for the NL Central lead and 1½ games ahead in the wild card.
"It's definitely an uphill battle," Aurilia said. "It's not like we're 4½ back behind one team; we're 4½ behind four (for the wild card). It's hard enough to catch one.
The Cubs' misery continued.
Wrecked by injuries, they were scrambling again when starter Juan Mateo left with tightness in the middle of his back after three hitless innings. He struck out three, walked two before being lifted for a pinch-hitter leading off the third.
It was the second time in four starts Mateo left with an injury. He took a line drive to the forearm by Pittsburgh's Jose Bautista on Aug. 30.
Mateo said through an interpreter that his back was bothering him in the bullpen before the game and it got worse. He said it's not serious, but was not sure if he'll make his next start.
"When I was throwing the ball, it was like pulling my back in. It kept hurting," he said.
Marmol came on to pitch in the fourth and got hit hard for the second straight day.
After allowing three runs in one-third of an inning in Thursday's 6-5 victory over the Los Angeles Dodgers, Marmol allowed four in his lone inning.
"You hate that early exit because you try to use everybody, but not overuse anybody for ensuing days," said manager Dusty Baker, who used four relievers.
Marmol walked the first two batters on nine pitches before Aurilia singled to left to drive in Norris Hopper with the game's first run. With one out, Ross sent a drive into the left-field seats for his 19th homer and a 4-0 lead.
"Our bullpen did a good job, except for a couple untimely walks, a base hit and a three-run homer," Baker said.
The offense did little, though.
The Cubs just kept lunging and looking at breaking balls, unable to mount a threat against Arroyo.
"He pitches backward," said Aramis Ramirez, who doubled. "When he's behind, he throws the breaking ball."
Acquired in a trade with Boston in late March, Arroyo went 9-3 in his first 15 starts before going 1-6 in his next 13. But in his last four outings, he has allowed four runs in 32 innings and lowered his ERA from 3.52 to 3.17.
"This game's funny," Arroyo said. "You saw the streak I went through in the middle of the year. Good, bad or ugly, I wasn't getting any wins. ... I started off really hot. I'm hot right now. I'm not doing anything special."
- Cubs 1B Derrek Lee missed the game to tend to a family emergency, leaving his status for Saturday's game in doubt.
- Although he's nursing a tight back, RHP Carlos Zambrano wants to start Sunday and Cubs manager Dusty Baker did not rule out. "I'm pitching on Sunday," Zambrano told reporters after the game, before walking away. "I'll see you Sunday, but I'll see you (Saturday) first."
- LHP Chris Michalak will start Saturday for the Reds against Rich Hill. He is 1-2 with a 5.09 ERA in five appearances and three starts.
- Cincinnati held Ken Griffey Jr. out of the lineup for the ninth straight game with a dislocated second toe on his right foot.