"He's got really good stuff," Cubs manager Lou Piniella said. "He knows how to win. A welcome addition."
With CC Sabathia going to Milwaukee, Harden was the best available pitcher, Cubs general manager Jim Hendry said.
"We felt that the two high end guys who would possibly be available were the two that have already been traded. Those are legit one and two starters. So we tried to focus on trying to acquire one," Hendry said.
"It certainly wasn't a reaction move. We would have gladly had Rich two or three weeks ago."
The Cubs began play Tuesday 3 1/2 games ahead of the Cardinals and four in front of the Brewers in the NL Central.
The Cubs can now match Milwaukee's one-two combination of Sabathia and Ben Sheets with All-Star Carlos Zambrano and Harden.
Athletics general manager Billy Beane said trading Harden doesn't mean his team is in selling mode. Oakland began the day six games behind the first-place Los Angeles Angels in the AL West and 3½ behind the Boston Red Sox in the wild-card race.
"I don't necessarily think we've taken a step back. I think we've taken a step forward for the next three to five years," Beane said. "We were balancing that the club's playing well but also keeping in mind that we've had numerous injuries over the last week. "
Hendry also talked with the Indians about Sabathia and said conversations with Beane about Harden have been ongoing for three or four weeks.
"I knew Rich was going to make his start Sunday, no matter what. At the same time, my chats with [Indians GM] Mark Shapiro, I think if he liked the deal he got he was going to try to do it before CC's turn. It had nothing to do with the timing," Hendry added.
"I heard it was going to happen. Still it's always a surprise," Harden said. "It's a bit of a shock but it will be good. They've got a good team and a chance of doing something special."
Chicago also got right-hander Chad Gaudin, who pitched previously for Piniella in Tampa Bay, and sent promising right-hander Sean Gallagher, outfielders Matt Murton and Eric Patterson and minor leaguer Josh Donaldson to the A's.
Harden, eligible to be a free agent after the 2009 season, is 5-1 with a 2.34 ERA in 13 starts this season. He's scheduled to make $4.75 million this season.
"We will have the rights for a year and a half, so that's the other advantage of him not being a free agent," Hendry said. "He has an option that could vest, or we could pick up the option or we could arbitrate. So he certainly has a chance to be a Cub through the end of next year."
The oft-injured Harden missed a month earlier this season because of a right shoulder strain. It was his sixth trip to the disabled list in his six-year career.
"Obviously there's some risk involved. He's missed some time but he's never had any surgery," Hendry said. "We did extensive work with their doctors."
The Cubs said they expected Harden to join the team Wednesday and pitch either Friday or Saturday at Wrigley Field against the San Francisco Giants.
"This gives us another weapon," Piniella said. "He'll fit in here really nice. Let's keep him healthy and pitching and go from there."
The Cubs won the NL Central last season but were swept out of the playoffs by Arizona. Without a World Series appearance since 1945 and without a title since 1908, they are obviously making a strong push. The team is also expected to be sold by the end of the season.
"We never looked at it as 'Oh, we're going to get in, we're better than last year, we'll take our chances if we get in,'" Hendry said. "There's a lot of baseball left. We don't look at it today like we're getting in for sure. ... Our mode all year has been that we will try to get better."
Gaudin was 5-3 with a 3.59 ERA in 26 games -- including six starts for Oakland.
"I had Chad at Tampa Bay and he's got good stuff," Piniella said. "A nice hard sinker and a nice slider so he'll fit in good in our bullpen. This was a move where we got a top-of-the-line rotation pitcher and a nice piece in the bullpen."
Murton was a former top prospect for the Cubs, but he's shuttled between Triple-A Iowa and Chicago the past two seasons. He's hitting .250 in 40 at-bats.
"Hopefully he'll go swing the bat for them," Piniella said.
Gallagher is 3-4 with a 4.45 ERA in 12 games, including 10 starts.
"It's a great opportunity for me to go over there and start every fifth day and keep going," he said.
Gallagher is expected to start at home Friday against the Angels.
"I'm excited about it," Athletics manager Bob Geren said of the trade. "We've made some great deals the last 12 months and this is another one. [Gallagher is] young and talented and we got a lot of other talent in the deal also. I think it's going to be another one of these trades where both teams are going to be real happy with it."
Patterson, the brother of Corey Patterson, was sent to the minors on July 3, and has also been on the frequent shuttle between the Cubs and Triple-A. Patterson was hitting .237 with a homer and seven RBIs in 38 at-bats with the Cubs. He also plays second base.
Donaldson, a catcher for Class A Peoria, was hitting .217 with six homers.
Murton and Patterson will be sent to Triple-A Sacramento. Donaldson may be moved up to Double-A or could be sent to Single-A Stockton.
Piniella said Hendry had told him recently that the Cubs likely didn't have the pieces to get Sabathia but that the club had other irons in the fire. He found out about the Harden deal when he arrived at Wrigley Field on Tuesday.
"There are some good teams in this division." Piniella said. "I like our team and I like it even more now."
Piniella said the trade should send a message to Cubs fans.
"This shows the Cubs are going to do everything in their power to get where we want to go," he said.
Information from The Associated Press was used in this report.