Right-hander Chad Gaudin, a versatile righty who started the season in Oakland's rotation but was bumped to the bullpen when Harden returned from his sixth trip to the disabled list since the start of the 2005 season, also was sent to the Cubs.
"I was kind of shocked," Gaudin told MLB.com by phone after the deal was announced. "Mostly because it was both Rich and me leaving. I felt like if he was gonna go, I'd go back into the rotation here."
In exchange for Harden and Gaudin, the A's acquired righty Sean Gallagher, infielder Eric Patterson, outfielder Matt Murton and catching prospect Josh Donaldson.
Gallagher, 22, is expected to start for Oakland on Friday against the Angels. Patterson, 25, and Murton, 26, will report to Triple-A Sacramento. Donaldson, 22, will be assigned to Class A Stockton or Double-A Midland.
"The Cubs were pretty aggressive," said A's general manager Billy Beane, who noted that he'd been discussing a Harden deal with Cubs GM Jim Hendry for several weeks. "Me and Jim talk quite a bit, and he'd been making pretty consistent contact about Rich. We finally reached an agreement this morning."
Harden, who hasn't made more than 19 starts in a season since 2005, spent a month on the DL with a muscle strain under his right shoulder early in the season but came back to post impressive numbers. He didn't suffer his first loss until his 13th start, last Friday against the White Sox, dropping his record to 5-1 with a 2.34 ERA
Although the A's have been limiting Harden to 100 pitches since his return, his velocity was down in each of his past two starts, and there were whispers that what Harden shrugged off as a something of a "dead-arm" phase was cause for greater concern.
That didn't stop the Cubs, who had personnel following Harden for some time, fueling the trade rumors.
"I had heard talk about it, but it's still always a surprise," Harden said just minutes after learning of the move. "It's a bit of a shock. But they're a good team and they have a chance of doing something special."
"They weren't looking for starting pitching," Beane said of the Cubs. "They were looking for Rich."
Beane said Gaudin wasn't always part of his talks with Hendry and conceded that it was difficult to part with the 26-year-old who can start and relieve.
"I think it's a good opportunity for Chad," Beane said. "Also, adding another Major League arm gives [the Cubs] a little protection going forward."
Gallagher began the season at Triple-A Iowa and was 2-2 with a 3.10 ERA in five starts before he was promoted to Chicago on May 3. He was 3-4 with a 4.45 ERA in 12 games, 10 starts, with the Cubs this year after posting an 8.59 ERA in eight relief appearances in his Major League debut last year. Gallagher was named the Cubs' No. 5 prospect by Baseball America prior to the 2008 season.
"Gallagher, I like him as a pitcher," Cubs manager Lou Piniella said. "He's a tough kid; he's got a good arm. I thought he came up here and pitched very well for us. I think he's going to get better with more experience."
Donaldson, ranked as the Cubs' No. 7 prospect by Baseball America, was Chicago's second-round pick in the 2007 First-Year Player Draft and batted .217 with six home runs and 23 RBIs in 63 games with Class A Peoria in the Midwest League this season. He hit .335 with nine home runs and 35 RBIs in 53 games in his professional debut last year.
Patterson was batting .237 with one home run and seven RBIs in 13 games over three stints with Chicago this year. He also has a .320 average with six home runs, 28 RBIs and 11 stolen bases in 52 games with Iowa and owns a .301 career average in the Minors with 124 stolen bases in 437 games.
Murton was batting .250 with no home runs and six RBIs in 19 games in two stints with Chicago and had a .298 average with a home run and 15 RBIs in 54 games with Triple-A Iowa. Murton made his Major League debut with the Cubs in 2005 and is a .294 career hitter in 308 games in the Majors.
In the wake of the trade, the A's recalled lefty Dallas Braden from Triple-A Sacramento and transferred Mike Sweeney to the 60-day DL.