"Primarily as we look at our team, our position players, and try to keep our offense moving forward, we wanted another big left-handed bat to grow with our core, ideally a first baseman," D-backs general manager Josh Byrnes said.
The 23-year-old Allen has spent time at Double-A Birmingham and Triple-A Charlotte this year. He has combined to hit .281 with 16 doubles, three triples, eight homers and 43 RBIs.
Allen, who was the White Sox fifth-round selection in the 2004 First-Year Player Draft, will be assigned to Triple-A Reno. The D-backs will make a move to call up a reliever before Wednesday's game.
"He's got a lot of power," Byrnes said. "He's a strong guy, he's athletic, he can actually run a little bit, he's gotten better defensively. You want a guy that can hit and hit with power, and he's done that -- he can do that."
The speed has been something Allen has worked on during his rise through the Minor Leagues.
"You don't want to be known as a base-clogger out there," Allen said this spring. "You want to be an athlete. That's what I try to do. Every year, I run the bleachers and work on my explosion. Speed can get you hits, even for a big guy like me. I take pride in my average speed that I possess."
Allen's defense is a work in progress, but Byrnes said that it has gotten better over time.
"He's an athlete with good work habits, so I think he's taking it seriously and knows that's important," Byrnes said.
Allen is a candidate for a September promotion when the rosters expand, and the team will also likely send him to play in the Arizona Fall League to evaluate his readiness for an everyday job in 2010.
While Pena had struggled recently, particularly when it came to getting left-handed hitters out, the club was hesitant to part with him because of his talent and also because he was not eligible for free agency until after the 2012 season.
But with the bullpen as a whole struggling, Byrnes decided to shake things up. Also, the GM wanted to take advantage of a trade market where, at the moment, the D-backs are one of the few clearly defined sellers.
With that in mind, Pena might not be the last player to be dealt before the July 31 non-waiver Trade Deadline.
"Our stated objective is pretty clear," Byrnes said. "We have a core of players that we don't really want to touch, we have some players we might consider if we get pieces back that have long-term value to us. Those discussions will go on. Whether they lead to more trades or not, I don't know."
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.