De Aza is expected to join the Red Sox for the finale of their four-game series against the Minnesota Twins on Thursday. He hit .214 with three homers and seven RBIs for the Orioles in 30 games this season before being designated for assignment May 27.
To make room for him on the 40-man roster, Boston transferred right-hander Brandon Workman from the 15-day to the 60-day disabled list.
Gunkel is 3-2 with a 2.90 ERA in Single-A and Double-A this year.
Boston also recalled left-hander Robbie Ross Jr. from Triple-A Pawtucket as the 26th player for their day-night doubleheader Wednesday against Minnesota.