The Reds placed Dunn on waivers last week, and the D-backs found out they had an exclusive 48-hour negotiating window with the Reds on Thursday. Because the rules state the 48 hours have to be working hours, the window carried over until Monday.
"I had a great time in Cincinnati, but a lot of good things come to an end," Dunn said. "I'm very excited about the opportunity to actually play on a first-place team in August. This is a huge deal for me to be able to play for a first-place team in August."
Dunn, 28, is tied for the Major League lead in homers with 32 to go along with 74 RBIs. The D-backs are counting on him to inject some life into an offense that has struggled since a torrid start in April.
Arizona is 10th in the National League with 534 runs scored and 12th with 113 homers.
"Obviously, we've been looking for more offense," D-backs GM Josh Byrnes said. "Adam is one of the premier hitters against right-handed pitching in all of baseball. Just from the offensive side of things he will give us a lift."
Defensively, the addition poses some challenges for manager Bob Melvin.
"Tomorrow he will be playing right field and hitting fourth," Melvin said of Dunn, who played left in Cincinnati this year and is not known as a great defender.
The D-backs' usual right fielder, Justin Upton, has been on the disabled list since July 9 with a strained oblique. He began an injury-rehab assignment Monday that could last up to 10 days.
Once Upton returns, how Melvin juggles the lineup will bear watching. Dunn has played first base in his career, and with second baseman Orlando Hudson out for the year with a fractured wrist, the club could conceivably put Dunn in left, Conor Jackson at first, Chad Tracy at third and Mark Reynolds at second. Or Dunn at first and Jackson in left.
"We're not going to get too far ahead of ourselves," Melvin said. "When [Upton] gets back we'll look at our options. Obviously, we have some moving pieces around here, whether it's Chad Tracy or Conor Jackson or whoever, and we'll continue to try and run the best lineup out there on a day-to-day basis."
Tracy, who underwent microfracture surgery on his knee during the offseason, has not played third this year and has been taking grounders there just once a week. Reynolds has not played second at the Major League level, though he did play the position in the Minors.
One option that Melvin ruled out was moving Upton from right field to second base.
For his part, Dunn was so excited about moving from a last-place to a first-place team he told Melvin he would catch if needed.
"Yeah, it's been a while since I played right field," Dunn said. "But I think it's one of those things that once I get out there during batting practice and get my feet wet, I'll be fine out there. Like I said, I don't care where I hit or where I play, I'm excited to get out there. I know they've got a young group of guys and a really good pitching staff. It's going to be sweet."
Dunn is in the final year of a contract that paid him $13 million this year. The D-backs and Reds will split what remains for this year, which is roughly $4 million.
If the D-backs do not re-sign Dunn in the offseason, they will pick up a compensation-round pick and either the first or second-round pick of the team that signs him. Byrnes didn't rule out bringing Dunn back, but it would seem to be tough to fit him into their payroll structure.
"Adam Dunn is at worst going to deliver us Draft-pick compensation, which is not insignificant," Byrnes said. "It's not as short-term oriented as maybe people feel. But the short term matters. We're in a pennant race, and we're adding one of the better power hitters in baseball, but there's a long-term side to this too."
Dunn, however is thinking only of the short term.
"I have not even thought about that," he said about being a free agent. "I don't think about what's going on down the road. I know I'm put in a great opportunity right now, and I'm going to make the most of it."
Buck was selected in the third round of the 2006 First-Year Player Draft and underwent Tommy John elbow reconstruction surgery last year. He returned to action a couple of months ago and went 1-4 with a 3.94 ERA in nine games (eight starts) for Class A South Bend. The right-hander was promoted to Arizona's high-Class A team in Visalia earlier this month, and in his lone start allowed three unearned runs in five innings.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.