Rosario was with Boston for all of five weeks, after the Red Sox claimed him off waivers from Miami on Oct. 17. The 27-year-old pitched in 10 games with the Marlins over the last three seasons, compiling a 15.26 ERA along the way.
The Dominican Republic native began the 2012 campaign at Triple-A New Orleans, posting a 1.04 ERA in 25 relief appearances and converting all 16 of his save opportunities, before earning a well-deserved promotion in June. He struggled, though, posting an 18.00 ERA in four appearances, before hitting the disabled list with a strained right quadriceps.
Rosario was deemed healthy by the end of August and optioned to Double-A Jacksonville at the time.
To clear a spot on the 40-man roster for him, the A's designated outfielder Jermaine Mitchell for assignment. Mitchell, a fifth-round Draft pick by the A's in 2006, hit .252 with six home runs, 15 doubles, 11 triples and 38 RBIs in 108 games with Triple-A Sacramento in 2012.