Bell's season with Miami was a rather rocky one, during which he struggled to maintain consistency and lost his closer role on multiple occasions. Bell and Marlins manager Ozzie Guillen were also caught in a public war of words, with Bell suggesting that Miami hire a manager "that everybody respects and looks up to" and Guillen claiming to not "respect [Bell] as a person."
In this multiple team deal, the Diamondbacks traded outfielder Chris Young and cash to the Oakland Athletics, getting shortstop Cliff Pennington and prospect infielder Yordy Cabrera. Cabrera and cash was then sent to Miami for Bell.
During the 2012 season, Bell saw action in 73 games and pitched 63.2 innings, earning a 4-5 record with 19 saves, 59 strikeouts, and a 5.09 ERA.