It appears the plan will be for Dunn to take over right field for now, a position he can pass at given his offense, despite the fact he hasn't played it in a couple of years. The acquisition could also be a slight hedge against Justin Upton not coming back strongly from his oblique injury. Upton is due to start his rehab assignment this week.
When Upton returns to the big leagues, things could shake up if the team were looking to get as much offensive punch as possible. There's at least a possibility that the club slides Mark Reynolds over to second base, a position he played in the minors. Chad Tracy would then move back to third, Upton would be back in right, and Conor Jackson and Dunn would then man left field and first base (or vice versa), with Augie Ojeda back on the bench.
The other scenario would be Upton on the bench the majority of the time, with Ojeda staying in the lineup, particularly with Brandon Webb on the mound. I wouldn't be surprised, however, if the D-backs shuffled positions, giving Reynolds and perhaps Dunn some extra position flexibility for their fantasy owners.
As far as Dunn's offensive output goes, it shouldn't change all that much. Though Chase Field's park factor this season favors hitters overall more than any other in baseball (with Great American ranking 18th), Dunn's old park was slightly more favorable for homers. The park effects should be negligible.
For the Reds, they will likely fill Dunn's spot in the lineup with Jerry Hairston, as it appears that his hamstring may have healed enough for him to return to the lineup this week. It should ensure that Jeff Keppinger and Corey Patterson continue to see regular playing time the rest of the season, although the Reds could take a chance and see what Chris Dickerson, a 26-year-old outfielder with a .384 OBP and 26 steals at Triple-A this season, has to offer. They could even use the wayback machine and call up Rob Mackowiak. Still, the best bet is for Patterson and Hairston to hold down the left and center spots, with Andy Phillips may start here and there.
In order to get Dunn for an end-of-season rental, the D-backs gave up 23-year-old right-hander Dallas Buck, a third-round pick in 2006 out of Oregon State. Buck underwent Tommy John surgery last August, and has thrown 50 innings at Class-A this season in his return, but is not full strength yet. When healthy, Buck has shown a good low-90s sinker, a quality changeup and a developing slider, and could be an interesting starting pitching prospect when his arm strength returns. He's likely at least a couple of years away from the big leagues. The Diamondbacks will also part with two players to be named later, presumably a couple of low-level minor leaguers.
Jason Grey is a graduate of the MLB Scouting Bureau's Scout Development Program and has won two Tout Wars titles, one LABR title and numerous other national "experts" competitions.