The bullpen was the most obvious area to address, and that is indeed where Mozeliak focused his attention. The Cardinals and Marlins had preliminary discussions about Mujica a week ago, though the deal wasn't officially made until Tuesday.
In exchange for the veteran righty, St. Louis sent former first-round Draft pick Zack Cox to Miami.
"We identified that we needed some bullpen help," Mozeliak said. "We identified some teams that might have that piece to match up with us and this is the deal that got done."
Mujica, 28, has a 4.38 ERA in 41 appearances this year, his second with the Marlins. Over his seven-year career, Mujica owns a 4.42 ERA. He has allowed 50 home runs in 348 1/3 innings and has 283 strikeouts.
He was expected to make the trip from Atlanta -- where the Marlins are playing a series -- to Denver on Tuesday night so that he can be available for the Cardinals on Wednesday.
To make room for Mujica, St. Louis optioned rookie reliever Trevor Rosenthal to Triple-A Memphis after Tuesday's 11-6 win over Colorado.
"We're excited to have Mujica, and I think he's going to be a nice fit," said manager Mike Matheny, who plans to plug Mujica into a sixth- and seventh-inning role. "We've obviously seen him a lot through Spring Training and through the season. The guy is going to bring in some experience and help out some of our young pitchers down there."
Adding to Mujica's appeal as a Trade Deadline target was the fact that he is under club control through the 2013 season. St. Louis will be responsible for paying Mujica the rest of the $1.625 million he is owed this year. Mujica will be arbitration-eligible over the winter.
Mozeliak's search for ways to upgrade the bullpen turned out to be more challenging than he initially anticipated. The relief market wasn't as saturated as it was expected to be, and those clubs that did have pieces to trade hardly lowered demands as the end of the month approached.
The Cardinals did not feel that their bullpen was so deficient that the club needed to give up top prospects to meet those asking prices.
"I thought there would be some more opportunities," Mozeliak said. "There would be guys to be had, but not always at a cost that we thought made sense."
The absence of additional deals does not, Mozeliak emphasized, suggest that the club considers a division title already out of reach. The Cardinals entered Tuesday trailing the Reds by seven games in the National League Central, but are only four games back in the race for one of two NL Wild Cards berths.
Rather, the tempered urgency to make additions came from a feeling that the Cardinals already have the pieces needed to get the club where it wants to go. The offense ranks first in the NL in batting average (.275) and runs scored (498). The rotation's 3.44 ERA is fourth-best in the league and the starters have been even more formidable in July.
That, coupled with left-hander Jaime Garcia's progress as he works to return from a left shoulder injury, made the Cards content with their depth.
"It just comes down to with what we have playing better baseball than what we've done," Matheny said. "You look at what we've got and I don't think it's a club where anybody expected wholesale changes. They did come up with a solid reliever for us because we knew that was an issue that we've had."
The cost for Mujica was Cox, the 25th overall pick in the 2010 First-Year Player Draft and a player who entered the season ranked as the organization's sixth-best prospect by MLB.com. But Cox's stock continued to fall as he climbed up the Minor League ladder.
He was not considered an above-average defender at third base, and Cox was batting .254 with a .294 on-base percentage in 84 games with Triple-A Memphis. With David Freese and Matt Carpenter sitting ahead of Cox on the organizational depth chart, it was hard to envision Cox breaking in with St. Louis anytime soon.
"It's never easy giving up somebody who was highly thought of at one time," Mozeliak said. "I thought we had some depth there [at the position]."
It's not inconceivable that the Cardinals are completely done adding, too. Last season, for instance, the club acquired left-hander Arthur Rhodes on Aug. 12. Completing deals in August, though, is more complicated.
Trades involving players on the 40-man roster cannot be made unless the players already have cleared waivers. If a player is claimed by one team, he cannot be traded. The club that placed the player on waivers can either withdraw the request and keep the player, or let the player go to the claiming team. If multiple teams put a claim on the same player, the club with a worse record gets the claim.
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.