To make room for De Aza on the 40-man roster, right-hander Brandon Workman was transferred to the 60-day disabled list.
De Aza was not activated for Wednesday's doubleheader against the Twins, manager John Farrell said, though he did not name the player involved in the acquisition.
Carlos Peguero, an outfielder acquired May 27 from the Texas Rangers for cash considerations after being designated for assignment, is expected to be the player dropped from the roster. Peguero had just three at-bats in three games for the Red Sox.
De Aza, 31, was designated for assignment by the Orioles last week after batting just .214 in 30 games. Originally signed by the Dodgers in 2001, De Aza has played for the Chicago White Sox, Miami Marlins and Orioles.
A career .265 hitter, De Aza started six games in the postseason for the Orioles last season, batting .333 (7-for-21) with three doubles. He opened the 2015 season as the team's everyday left fielder and also started eight games in right, but he eventually lost playing time to outfielders Travis Snider, David Lough and Steve Pearce.
With Daniel Nava on the disabled list with a strained left thumb, De Aza gives the Red Sox an experienced outfielder and left-handed bat who has played all three outfield positions and gives Farrell some flexibility.
Nava is hitting .159 with a .440 OPS in 27 games. He is 4-for-38 (.105) in his last 16 games dating to April 23.
Farrell said Nava, who was placed on the DL on May 28 (retroactive to May 26), has yet to swing a bat.
"An MRI confirmed a bruised muscle in his left hand, up from the thumb toward the palm area,'' Farrell said. "There's inflammation there that they are trying to calm down and heal. I don't have a date he'll have a bat in hand.''
Farrell said De Aza provides depth at least until Nava and Shane Victorino (hamstring) return.
"We didn't find anything out in [Nava's] exam that this is going to be longer than we thought,'' Farrell said.
Information from ESPN.com's Jerry Crasnick was used in this report.