But with longtime teammate Ken Griffey Jr. already gone and the Reds on a six-game losing streak, Dunn didn't mind packing his bags and heading West.
"It was one of those pleasant surprises," the slugger said on a conference call. "To come off the terrible month that we had in Cincinnati, and you get a phone call today saying you're being traded to a first-place team. As a player that's all you could ask for."
Dunn, who was tied for the big league lead in homers, could boost an Arizona offense that was ranked 20th in the majors in runs scored. The Diamondbacks (60-58) entered the day with a 1½-game lead in the NL West over the Los Angeles Dodgers, who recently traded for Manny Ramirez.
Arizona sent minor league right-hander Dallas Buck to Cincinnati (52-67) along with two players to be named. The 23-year-old Buck was 1-4 with a 3.94 ERA at Class-A South Bend.
Dunn, 28, was batting .233 for the Reds with 32 home runs and 74 RBIs. He has a good eye at the plate (80 walks) but also strikes out a lot (120 Ks).
Dunn has a .247 career batting average with 270 homers and is eligible for free agency at the end of the season.
Dunn had played his entire career with the Reds and is nearing the end of an $18.5 million contract. He spoke with Arizona manager Bob Melvin but isn't sure what role he'll fill.
"I'll do whatever," Dunn said. "If they want me to catch, I'll catch."
Buck got a call from D-backs farm director A.J. Hinch, telling him he had been traded for Dunn. Buck, who was traveling with the Single-A Visalia Oaks, was already on the bus to San Jose, Calif., when Hinch called him. He had to tell teammates of the trade en route.
"It was kind of funny," Buck said, "because I was rumored to be in a deal for Teixeira, so when I just told them they all thought I was messing with them."
Buck was drafted by Arizona in the third round of the 2006 draft. Once a top-notch prospect, Buck's draft status fell after an elbow injury revealed on the eve of the '06 draft bumped him out of a likely first-round slot. He had Tommy John surgery last summer and had begun pitching in Single-A this summer.
On July 31, Cincinnati sent Griffey to the Chicago White Sox for reliever Nick Masset and a Triple-A player. On Sunday, the Reds designated veteran catcher David Ross for assignment and called up catcher Ryan Hanigan from Triple-A Louisville.
Reds manager Dusty Baker, whose team has lost nine of 10 games since the Griffey trade, said he'll miss having Dunn on the club.
"It's tougher to lose a good guy, you know, than the 40-home run guy," Baker said. "It's going to be tough to replace his offense, but it's even tougher to replace the man."
Information from ESPN.com's Amy K. Nelson and The Associated Press was used in this report.