It was a move made to make room for Triple-A Pawtucket call-up Ryan Kalish. Over The Monster has more details:
Ranked as the second best prospect in the system by SoxProspects.com, Kalish is something of a jack-of-all-trades in the outfield. Capable of playing center or an above-average corner outfield, Kalish is an all-around hitter who combines good power and contact hitting with a fantastic approach at the plate. He can be a threat when he gets on the basepaths, with 25 steals in 28 attempts.
To make room for Kalish, Jeremy Hermida has been designated for assignment. Having been brought in by the Red Sox as a high-upside fourth outfielder, Hermida has done nothing but show exactly why he's considered a bust, displaying awful plate discipline and even worse defense.
Additionally, the Red Sox called-up LHP Dustin Richardson.
Kalish was in the starting lineup for Saturday's game, his MLB debut.