Duke is scheduled to join the Cardinals ahead of Tuesday's series opener in Cincinnati.
"I think yesterday we felt that getting Zach Duke was something we needed to do and it addressed a hole that we felt we needed to fill with what's been going on in our bullpen," general manager John Mozeliak said Monday evening. "As the clock ticked and we looked at different opportunities today, we never felt like we got to the point where it made sense for us. I certainly understand that people are always looking at the Trading Deadline as this great opportunity to do something. I feel like when you look at our club, even with the injuries, we still have all the right pieces to be successful and really didn't feel there was an incremental upgrade that made sense."
Mozeliak noted that in exploring other trade options, many clubs pursued the Cardinals' best prospects, among them Alex Reyes (No. 1) and Luke Weaver (no. 2). It was a cost the Cardinals weren't willing to shoulder.
"There could be some long-term ramifications in terms of the talent or young talent that you have to give up in those scenarios," Mozeliak said. "Maybe today you don't feel like you've given up much but in the out years it's something you come to regret."
That left the Cardinals adding only Duke, who made an American League-most 53 appearances with the White Sox. The Cardinals intend to utilize him in a setup role to complement Seung Hwan Oh, Kevin Siegrist and Jonathan Broxton. He's under contract for 2017, too, a factor that Mozeliak said was weighed heavily before making this deal.
Mozeliak has seen similar midseason deals for relief help pay off in big ways before. The Trade Deadline acquisitions of Octavio Dotel and Marc Rzepczysnki helped stabilize a bullpen that played a key role in the team's world championship run in 2011. A year later, the Cardinals swung a midseason deal for Edward Mujica, who later became the team's closer. Last summer, the club acquired Broxton and Steve Cishek at the Deadline.
The Cardinals did not, Mozeliak said, make a last-minute dive into the infield market after learning on Monday that Aledmys Diaz will be out for several weeks with a hairline fracture in his right thumb.
While Duke will be the only new face on the Cardinals' roster this week, the roster is nevertheless positioned for a slew of changes. The Cardinals anticipate activating Brandon Moss and Jhonny Peralta from the 15-day disabled list on Tuesday. Matt Carpenter is likely to rejoin the big league club on Friday. Corresponding moves have not yet been announced.
Jenifer Langosch has covered the Cardinals for MLB.com since 2012, and previously covered the Pirates from 2007-11. Read her blog, follow her on Twitter, like her Facebook page and listen to her podcast. This story was not subject to the approval of Major League Baseball or its clubs.