Rosenbaum, who started 15 games in Boston's organization last season, will be used as a reliever in 2016, the Boston Globe reports.
The 28-year-old left-hander was recovering from Tommy John surgery when he was traded to Boston at the beginning of 2015. Rosenbaum went on to post a 5.81 ERA in 15 starts between Short-Season Lowell and Double-A Portland last year. Boston manager John Farrell sees his low-90s fastball playing up out of the bullpen. Rosenbaum also features a changeup, cutter, and sinker.