"I was pretty fired up for this one," Wood said. "We took a game plan out there, and it was working for a little bit, then it didn't. I just didn't make the adjustment soon enough."
Wood cruised through the first three frames, allowing just two singles while recording three strikeouts. A chaotic error chalked up to Wood on a play at first base allowed a run to score in the second before things fell apart for the 25-year-old lefty in the fourth.
Four straight Reds reached base off their former teammate, including bookend home runs from Ryan Ludwick and Todd Frazier surrounding a plunking of Jay Bruce.
The Reds then strung a two-out rally to add two more runs following three straight extra base hits from Arroyo, Zack Cozart and Drew Stubbs, to put the Reds ahead, 7-3.
"The doors fell off," Wood said. "Ludwick, first-pitch swinging hit a homer. I wasn't really worried about it. It was just a little homer. Then I end up hitting Bruce and then Frazier got me. Then things just went downhill. They put some good swings on pitches that I missed and hit them out the park."
Bruce clocked a solo home run off Wood in the fifth, giving the first-year Cub his ninth loss of the season.
Wood faced his former club less than a week ago on Saturday in Chicago with much better luck. He lasted seven innings, giving up just one run on five hits without receiving a decision.
"I think nature itself takes over in a game like that against your old team," Cubs manager Dale Sveum said. "You're always going to spend more energy and do more than is probably in there just to get the job done. He had a heck of a game last time against them doing it.
"He was out there in a gun fight with a knife tonight."
Wood is now on a six-game losing streak, with a 7.68 ERA in that span. He has allowed six or more earned runs in four of his past eight starts, and now holds a 5.25 ERA against the Reds this season.
The Cubs were in the driver's seat early, boosted by a solo home run from Luis Valbuena in the second and two more runs in the top half of the fourth to put them ahead, 3-1.
That was when the Reds began laying the wood on Wood.
"It didn't look too good there for a while, but we answered back when they scored," said Reds manager Dusty Baker. "We had a big five-run inning and guys were on Woody pretty good in that inning. Woody had us eating out of his hands over there in Chicago, and he wasn't quite as sharp tonight as he was over there in Chicago."
The Cubs matched the Reds' 10 hits with 11 of their own, but stranded 10 runners on base compared to the Reds' three.
"We got a lot of hits tonight, but that team had a very good bullpen," said Starlin Castro, who was 2-for-4 with a run scored Friday night. "If you want to do something, you need to do it to the starting guy. The bullpen is too good."
The Cubs' bullpen had something to hang their hats on, too.
Michael Bowden and Alberto Cabrera came on in relief for Wood, tossing a combined three innings and giving up just one total hit, blanking the Reds for the latter part of the contest.
"It was nice to be very efficient and be available for tomorrow, too," Sveum said. "Bowden and Cabrera did a great job because they'll both be available for both games tomorrow now, too."
With a doubleheader slated for Saturday, the Cubs may need every arm in the 'pen to last all 18 innings.