The Red Sox said Thursday that the struggling Masterson has right shoulder tendinitis. In seven appearances this season, he is 2-2 with a 6.37 ERA. On Tuesday night, he gave up six runs on six hits in 2 1-3 innings in a 9-2 loss to the Oakland Athletics.
In the offseason, Boston signed Masterson as a free agent from the St. Louis Cardinals.
Ross began the season with Boston and was 0-0 with a 6.17 ERA in 13 outings before being optioned to Triple-A Pawtucket last Sunday.