Boston, which in return for De Aza and $650,000 will receive minor-league pitcher Luis Ysla, acquired De Aza from the Orioles this past June. He is hitting .261 with seven home runs and 32 RBIs on the season, including a .292 average since joining the Red Sox.
De Aza, 31, will join a Giants outfield that is missing Hunter Pence (oblique strain) and Angel Pagan (knee tendinitis). San Francisco tried to shore up its outfield on Aug. 20 when it acquired Marlon Byrd from the Reds, but will add the veteran De Aza to help spell current starters Byrd, Nori Aoki and Gregor Blanco.
The Giants, who opened up a three-game series with the Dodgers on Monday, entered the day 3 1/2 games back of Los Angeles in the NL West and 5 1/2 games back of the second wild-card spot.