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. He made his Major League debut on April 17 against the Blue Jays, going 1-for-2 with a walk.
The versatile 23-year-old was hitting .333 in 24 games for Pawtucket, making 11 starts at second, eight at shortstop, three at third and one in the outfield.