The Red Sox designated right-hander Sean O'Sullivan for assignment on Monday and recalled infielder Marco Hernandez from Triple-A Pawtucket.
O'Sullivan, 28, was 1-0 with a 7.94 ERA in three appearances (two starts) for the Red Sox this season after signing as a Minor League free agent in December. He did not factor in Sunday's decision after starting and giving up six runs (five earned) in 4 1/3 innings.
Hernandez, who will join the Red Sox for Monday's series opener against the Royals, will be embarking on his third stint with the big league club this season.