The Cardinals purchased the contract of the 36-year-old Fuentes from Double-A Springfield in time for Wednesday night's game against the Dodgers.
Fuentes is a four-time All-Star with 224 career saves to lead active left-handers. He led the majors with 48 saves in 2009 with the Angels and had six seasons with 20 or more saves.
"I think he definitely can be an asset," Matheny said. "He's funky. It wasn't too long ago he was throwing really hard with incredible deception.
"He hides the ball and basically steps into the dugout and throws it across the plate."
The 36-year-old Fuentes said he didn't care when he pitched. He likes the look of a team that was in third place in the NL Central, six games out of first, but had won four of five.
"I knew there was an opportunity here, a spot open in the bullpen or at least a need for another back-end guy," Fuentes said. "It just seemed like the right fit."
The Cardinals had also been in the bidding for Fuentes before the '09 season.
"It was real close," Fuentes said. "It was my first time in free agency and there's a lot to take in, a lot to learn."
Fuentes was 2-2 with a 6.84 ERA and five saves in eight chances in 26 appearances for Oakland before being released July 11. The Athletics had signed Fuentes to a two-year, $11 million contract in January as a setup man.
Fuentes blamed control woes on his problems in Oakland. Over his last seven outings he was 0-2 with a save, two blown saves, a 19.50 ERA and .452 opponents' batting average.
"I was falling behind hitters and when I did come into the strike zone it was kind of the fatter parts of the plate because I had to throw a strike, and guys were jumping on it," Fuentes said. "It was just kind of an array of different things. It goes downhill quickly when you don't throw strikes."
Earlier this season, Fuentes became the first Oakland reliever ever to take the loss in four consecutive appearances.
Struggling Cardinals right-hander Victor Marte was optioned to Triple-A Memphis and right-hander Chuckie Fick was designated for assignment to clear space on the 40-man roster. Fuentes will be the third left-hander in the bullpen along with Marc Rzepczynski and rookie Barret Browning.
Rzepczynski has been a bit of a disappointment with a 4.96 ERA and Browning has thrown just 7 2/3 major league innings.
The 31-year-old Marte was 2-2 with a 4.46 ERA. He allowed runs in five of his last nine outings, surrendering five runs in 6 1/3 innings with a blown save and a loss.
"Vic was really a big part of this bullpen for the majority of the season," Matheny said. "He just kind of ran into a bad spot. Lately, he's been finding the good part of the bat."
"I don't think it's going to have any effect at all once the bruise calms down," Berkman said. "But it's a bruise, you all, it's not like it's a major deal."