Duke was acquired by the D-backs last week from the Pirates for a player to be named.
At the time of the deal, Arizona general manager Kevin Towers said the team wanted Duke to be part of its 2011 rotation, but only if it could reach a contract agreement with him before Thursday's 10 p.m. MT deadline to tender players contracts.
As of Wednesday, prospects for a deal weren't promising, but on Thursday afternoon there was momentum toward a deal.
"We're excited," Towers said. "He's a lefty with a lot of experience that's still in the prime of his career. We think there's a lot of upside there."
Duke, 27, made $4.3 million last season while going 8-15 with a 5.72 ERA. While the D-backs see him sliding nicely into the back end of their rotation, they were not about to risk going to arbitration and having him receive a significant salary bump.
The D-backs' rotation now consists of Joe Saunders, Ian Kennedy, Daniel Hudson, Barry Enright and Duke.
"It kind of fills out our rotation, which is kind of nice this early in the offseason," Towers said.
That being said, Towers did not rule out trying to add another starter this winter.
"We feel pretty strong about the five, but that's not to say we wouldn't add somebody else that could be a spot starter, long reliever, but I would say it's probably not a priority now," Towers said. "I think our focus is now more addressing the bullpen now that we have Duke."
Providing innings is usually not a problem for Duke, who tossed 215 1/3 innings in 2006, 185 in 2008 and 213 in 2009. Last season, while making 29 starts, he threw 159.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.