Fuentes will not join the big league club immediately, but instead will report to the team's Gulf Coast League affiliate on Sunday to begin a predetermined seven-day Minor League program. That will give Fuentes, 36, time to shake off some rust (he hasn't pitched since June 30), as well as time to work on a few mechanical adjustments.
"He wants to get himself right," Cardinals general manager John Mozeliak said. "I think he's approaching it with the right attitude. Even if he can get himself partially back to where he can help add depth -- not only from the left side, but also to be able to get right-handers out -- that would be important."
The Cardinals could extend Fuentes' Minor League stay through the end of the month, if they think that's best. Fuentes has a July 31 out clause built into his contract that would allow him to ask for his release if he has not been called up to the Majors by the end of the month.
"We're going to have to see how he comes along," Cardinals manager Mike Matheny said. "I remember who he used to be, and that may have been the only lefty I never liked to face. I know his stuff is a little different now, but I also know he has something to bring to the table."
The organization has been looking for left-handed relief help since J.C. Romero turned out to be ineffective, and was interested in infusing some veteran leadership into the 'pen. The Cardinals entered the season anticipating that Romero and Scott Linebrink could provide some of that experience. Both have been released.
Fuentes, who was released by the A's earlier this month, began the 2012 season well. He allowed just six earned runs in his first 19 innings, with three of those runs coming in one April 21 appearance. Then it all fell apart.
The lefty was scored upon in six of his final seven games with Oakland and surrendered 13 runs in that six-inning span. That led the A's, who owe Fuentes the rest of the $6.5 million he is due this season, to part ways with him. The Cardinals do not believe that Fuentes' sudden decline in effectiveness has anything to do with an injury.
"His velocity was down a little bit, but he feels like he's got more in the tank," Mozeliak said. "Hopefully, if we can make some small adjustments with him and get him back to where he has that arm strength, there might be some lightening in a bottle for us."
While Fuentes' fit won't be defined until he arrives in St. Louis, he'll likely fill the lefty-specialist role. Left-handed batters hit .265 off Fuentes this season, while righties hit him at a .304 clip.
"I look at him, right now, as adding depth to that sixth or seventh inning that we know we need to do," Mozeliak said. "How he gets used or defined from there, I think time will tell."
Fuentes had other offers to consider, but ultimately weighed the chance to pitch for a contender most heavily. The four-time All-Star last pitched in the postseason with Minnesota in 2010.
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.