McCarthy signed a minor-league contract with the Red Sox on Thursday, Bob Nightengale of USA Today reports.
McCarthy became a free agent at the end of October rather than accepting an outright assignment from the Royals, and he'll now work out of Boston's minor-league system. The 28-year-old has made major-league appearances with the Royals in each of the last five seasons, and he posted a 4.48 ERA and 1.52 WHIP over 66.1 innings during the last two seasons.