Adam Dunn is 28 years old, a pending free agent who leads the National League in home runs and ranks second in walks. Yesterday, the Cincinnati Reds paid $2 million for the right to trade him to the Arizona Diamondbacks, a team that started the day just 1.5 games ahead of the Los Angeles Dodgers in the standings and needed, more than anything, a power hitter with a good on base average. Let no one say that Cincinnati general manager Walt Jocketty isn’t a gentleman, or that Arizona GM Josh Byrnes doesn’t know a good thing when he sees it.

If this trade doesn’t quite rate with the coup that landed Manny Ramirez in Hollywood, it should still nearly zero out the advantage the Dodgers picked up in that deal. No one is going to match Ramirez on a contract drive in the middle of a pennant race; the man has been hitting every other pitch he sees on the screws since arriving on the West Coast, and he’s not going to stop. Dunn, though, will come as close as any hitter Arizona could conceivably have acquired.

Arizona ranks 10th in the league in runs scored despite playing in one of the better hitters’ parks in baseball, and much of the problem is their outfield. Center fielder Chris Young is a brilliant defender, but for the second straight year his on base average is listing along below .300. Twenty-year-old right fielder Justin Upton stands a very good chance of someday being baseball’s second $200 million man, but since the beginning of May he’s hit .193, and he’s on an injury rehab assignment in Triple-A right now. Eric Byrnes, Alex Romero, Jeff Salazar, and Chris Burke have hit a collective .211 in 563 at bats, nearly all in the outfield.

Dunn, who ranks fifth in the majors in home runs and seventh in walks since 2001, isn’t really a great hitter. He ranks just 11th in slugging average and 17th in on base average despite playing in a park nearly as good for left-handed power as Arizona’s, because he’s one of the worst of all time at putting the bat on the ball. He can’t hit a changeup, and his preposterous strikeout totals — he’s the only man ever to whiff more than 190 times in a season twice — keep his batting average comically low. (He’s hit .247 in his career, and .233 this season.) Nor does he much make up for his flaws with other elements of his game. An enormous man who once played quarterback at the University of Texas, he’s indifferent in left field and at first base and, at best, an average base runner.

Still, he immediately steps in as Arizona’s best hitter and an enormous improvement on Romero and Burke, whose at bats he’ll be taking for right now. He might even be expected to improve now that he’s away from the Reds. Their manager, Dusty Baker, is one of the best in the game at working with and improving right-handed line drive hitters who neither walk nor strike out much. Unfortunately, he tries to make everyone hit like that. Away from the Reds, Dunn will likely hit fewer balls in the ground and more in the air, meaning he’ll put more in the seats — the best outcome of all. The one thing the Diamondbacks haven’t had is a true lineup anchor; they have that now.

Whatever effect Dunn has on the NL West race — and by himself he may represent the difference between the Diamondbacks holding off the Dodgers and failing to do so — this will likely prove a terrific deal for Arizona. In addition to getting the Reds to pick up half of the $4 million left on his salary, they seem to have given up little enough talent. They sent out 23-year-old pitcher Dallas Buck, who has yet to pitch above A-ball, and will add two players to be named later. Neither, it’s almost certain, will be near the quality of the prospects the Dodgers shipped off for Ramirez.

In exchange, Arizona not only gets a player who’s on his way to his fifth straight season of 40 home runs with 100 walks, but they get a chance to get a close-up look at a player who, if they sign him to an extension, could help power their offense for years. With his low batting average, occasionally leaden defense, and reputation for apathy, Dunn gets about as little respect as possible for such a productive player. Right now, in the cauldron of a race, he has a chance to show up everyone who ever had a bad word to say about him, and maybe even lock down a contract worthy of some of his gaudier statistics. Whether he can take advantage will determine not just Arizona’s fate, but his own.

tmarchman@nysun.com