How do you improve the postseason chances of a team that’s 18th in slugging percentage and and 21st on on base percentage? Answer: You trade for Adam Dunn.

That’s exactly what the Arizona Diamondbacks did today, acquiring the lefty slugger for prospect Dallas Buck and two players to be named later.

It’s no secret that Arizona was in dire need of a bat. With Orlando Hudson now out for the remainder of the season, first baseman/left fielder Connor Jackson was the only big offensive threat in the entire lineup. With the left fielder Dunn joining the team, the Diamondbacks can keep Jackson at first for the rest of the season. Moreover, this allows manager Bob Melvin to take Chad Tracy out of the everyday lineup and use him as a lefty bat off the bench or to spell Jackson or third baseman Mark Reynolds on occasion. The D-Backs will also be getting RFer Justin Upton back from the DL any day now, and should also help give the club a nice offensive boost.

Prior to the trading deadline, SI’s Jon Heyman was reporting that Arizona had at the time offered Tracy to Cinci for Dunn but was unsurprisingly rejected. A couple of weeks and a Manny-to-Dodgers trade later, the Diamondbacks agreed to give up 23-year old Dallas Buck, a righty pitcher with High-A Visalia. Buck was drafted in the third round back in 2006, but underwent Tommy John in 2007 and missed the rest of the season plus the bulk of this one as well. He began his 2008 campaign in Low-A South Bend and had made one start in Visalia thus far. As such, I love this move from Arizona’s perspective. In the deep, prospect-laden Arizona organization, Buck was never considered one of the top prospects. And coming off a major surgery, Buck had missed a great deal of development time for a pitcher his age.

On the Cincinatti side, this one’s a bit tough for me to evaluate. One could argue that they needed to get something for Dunn, who is an upcoming free-agent. However, a hitter of his stature would have probably netted a nice compensation pick or two once he signed elsewhere. So the key becomes who those two “players to be named later” are. And without knowing what kind of offers the Reds had been getting for Dunn prior to the deadline (as I’ve written here before, I’ve never quite understood why there was so little interest in him from contending teams), I don’t know what they missed out on.

But I do think that the race between Arizona and LA just got a little bit more interesting.