Willingham, 31, joins newly signed free-agent designated hitter Hideki Matsui, as the A's moved this week to bolster a lineup that didn't do enough to support a pitching staff that posted the lowest ERA in the American League last season.
"Our motivation is the continued desire to upgrade our offense, particularly from the right side of the plate," A's general manager Billy Beane said. "We just thought Josh was too good of a player to pass up at this juncture."
Willingham brings some right-handed pop, having averaged 20 home runs and 66 RBIs over the past five seasons for the Marlins and Nationals. Kevin Kouzmanoff led the A's with 16 home runs last year.
Willingham hit .268 with 16 home runs and 58 RBIs in 114 games for the Nationals in 2010, before missing most of the last month and a half with a left knee problem. He underwent surgery in mid-August and said he's fully recovered now and has been back on his normal workout routine since October.
"I'm excited because I know the potential this ballclub has," Willingham said about an hour after learning of the trade. "I know they have really good young pitching and have already upgraded their lineup a lot. So I'm excited to be part of that. I think this team is ready to win."
The A's acquired outfielder David DeJesus from the Royals earlier this offseason and now add Willingham to the returning mix of Coco Crisp, Ryan Sweeney and Conor Jackson. Willingham has played almost exclusively in left field in his career, with 560 games in left as opposed to 34 in right field.
Beane envisions Willingham filling the same left-field role for the A's, with DeJesus having the versatility to play right field. He also said the deal doesn't mean the club has lost any confidence in Sweeney, but speaks more to the desire to add depth to avoid the injury problems that built up last year.
"Ryan had trouble staying healthy the last couple years, but he's a good hitter and very much in our short- and long-term plans," Beane said. "In a positive sense, we've got what we think are good Major League players who deserve to play, and the trick will be getting them all enough at-bats to stay productive and also stay healthy and fresh for a 162-game season.
"We still feel highly about Ryan and will continue to do so despite this trade."
Matsui also played left field earlier in his career, but he's targeted strictly at DH in place of Jack Cust, who signed with Seattle last week.
Willingham is a career .265 hitter with 103 home runs and 336 RBIs. He became the 13th player in Major League history to hit two grand slams in one game -- and seventh to do it in back-to-back innings -- on July 27, 2009, against the Brewers in a 14-6 victory at Milwaukee. He finished that season hitting .260 with 24 home runs and 61 RBIs.
The 6-foot-2, 215-pound Alabama native earned $4.6 million last year and is likely in line for a pay hike going into his third and final season of arbitration eligibility. He'll become a free agent in 2012, though Beane said he'd talk to his agent soon about whether a contract extension might make sense for both parties.
The A's have been interested in Willingham for several years and Beane said talks kicked up when the club was pursuing DeJesus earlier in the offseason, then resumed in the past week. Beane likes not only the long-ball potential, but Willingham's career .367 on-base percentage.
Willingham has never played in Oakland, but says he looks forward to his first AL stint and isn't worried about the ballpark's reputation.
"I don't think I've ever really played in a hitter's ballpark in my career," he said. "Florida isn't really a hitter's park and Washington played really fair. In my experiences, I'm going to pull most of my home runs. They'll go out of anywhere, anyway."
Beane said it wasn't easy giving up the hard-throwing Rodriguez, 23, who pitched 29 games in relief for the A's last season with a 4.55 ERA in 27 2/3 innings.
"There's no getting around the fact he's a unique talent with a 100-mph fastball," Beane said. "But we knew we'd have to give up something and felt at this time the attraction of bringing Josh on board was not something we can pass on. He is a unique talent though, and we're prepared for him to go on and have an outstanding career."
Brown, 25, was a first-round sandwich Draft pick in '07 out of Oklahoma State, but has yet to reach the Major Leagues.
He hit .320 with 10 home runs and 49 RBIs for Double-A Midland last year and was named to the Texas League All-Star team, but batted just .193 with five home runs in 41 games after a late-season promotion to Triple-A Sacramento.
The A's have been busy this week. In addition to signing Matsui and trading for Willingham, they also came to contract agreements with veteran free-agent pitchers Brandon McCarthy and Rich Harden.
Beane said the team may still pursue bullpen help to make up for the loss of Rodriguez, but sounded as if his offensive makeover is likely complete.
"At this time, there are obviously a lot of guys off the board," he said. "That's one of the reasons we jumped on Josh when we did. You see the dwindling opportunities, and if we're going to get another impact bat, it was unlikely we could get one going forward. But we'll keep our eyes and ears open."
How much improved is the lineup with the addition of Willingham, Matsui and DeJesus?
"We think we're better," Beane said. "If anything, we're deeper and more balanced. Are there some shortcomings? Sure, but that's going to happen in this marketplace. We're not going to have the perfect club with our payroll capabilities. But it is better and it'll also be deeper with some guys coming back."