Cervenka, 26, has a 3.18 ERA and 35 strikeouts in 34 innings this season. He has held left-handed hitters to a .143 batting average, while right-handers are hitting .212 against him.
"What he's been this year is pretty much a situational guy," Marlins manager Don Mattingly said before Saturday night's matchup against the Rockies. "He gets lefties out. He's had a lot of success with that."
For most of the season, Mike Dunn has been the only left-hander in the Marlins' bullpen.
"There are times it can be very nice," Mattingly said of having two left-handers in the 'pen. "With a team like the Rockies, they've got a number of lefties in the lineup. …There are times early in the game -- sixth inning or fifth inning -- that you're trying to get out of a situation. You may need to get that one guy out, so it can be very nice to have one."
The Marlins have not opted for many lefty-on-lefty matchups in the last three innings this season, instead rolling with their most reliable relievers. None of right-handed relievers David Phelps, Kyle Barraclough, Fernando Rodney or A.J. Ramos have significant righty-lefty splits.
Cervenka does miss plenty of bats with 9.26 strikeouts per nine innings and a 77 percent contact rate, but he has issued 6.09 walks per nine innings, which is third-highest in baseball among pitchers with at least 30 innings pitched.
"In talking to our people, obviously we've seen a lot of him being in our division," president of baseball operations Michael Hill said. "We've done our video work on it. [Pitching coach] Juan [Nieves] and Jim Benedict, our VP of pitching development, feel like there are some things we can address with him that hopefully allows him to be more consistent in the strike zone because when he is, he's very difficult to hit and has had success both against right-handers and left-handers and we're happy to be able to work a deal out and bring him aboard."
Seymour, 21, was a seventh-round Draft pick in 2014 and was the Marlins' No. 8 prospect, according to MLBPipeline.com. He stole 36 bases in 103 games at Class A Greensboro this season, batting .252 with 22 walks and 96 strikeouts.
Mader, 22, was a third-round Draft pick in 2014 and was Miami's No. 24 prospect. He pitched in 22 games for Class A Advanced Jupiter, starting 21, and went 7-6 with a 3.50 ERA and 81 strikeouts in 103 innings.