As soon as the trade was made, Duncan was optioned to Triple-A Pawtucket.
The Red Sox will pick up just about all of the balance on the $13.5 million Lugo is owed through the end of next season.
Duncan -- the son of Cardinals pitching coach Dave Duncan -- had been optioned to Triple-A Memphis by St. Louis just hours before he was traded.
He will give the Red Sox depth in the event they suffer injuries in the outfield. Duncan, 28, hit .227 with five homers and 32 RBIs in 87 games for the Cardinals. He had 41 walks.
Duncan, 28, underwent a rare surgical procedure over the winter, having a titanium disk inserted in his neck. He started the season well, hitting .304 and slugging .522 in April. Since then, though, he has steadily trended downward. In May, Duncan hit .227 and slugged .386, and in June, his slugging percentage fell all the way to .289. Duncan is 1-for-31 since June 29.
Duncan is a career .257 hitter, with 55 homers and 175 RBIs in 389 Major League games.
Lugo hit .284 with one homer and eight RBIs in 37 games for the Sox, and he had noticeably diminished range at shortstop.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.