The 27 year old was signed originally as an international free agent out of the Dominican Republic in 2004 and appeared in just four games for the Marlins this past season. Over three innings of work he allowed 6 runs on just 8 hits. While he showed fair control (zero walks allowed against 25 batters faced), he was awfully hittable (2.667 WHIP). Rosario fared much better in the minor leagues, posting a 1.99 ERA and 0.979 WHIP to go along with 17 saves over 31.2 innings of work across three levels.
Across the past seven seasons in the Marlins minor league system Rosario has shown some promise, posting a 21-14 record and 3.48 ERA over 310.0 innings, adding 9.3 K/9 and 2.1 BB/9.
Presumably Boston is viewing Rosario as another potential option out of the bullpen next season, though it is worth noting that Rosario still has a minor league option remaining so he can spend some additional time in Triple-A if needed. Ranked by Baseball America as Miami’s 29th best prospect before the 2012 season, Rosario profiles no higher than a middle reliever so his loss may not prove to be a significant one in the long term for the Marlins.