As currently constituted, the Giants have no left-handed hitters on the bench when Gregor Blanco starts in the outfield, other than switch-hitters Ehire Adrianza and, when he's activated Tuesday from the disabled list, Angel Pagan.
De Aza, 31, hit .292 in 60 games since the Red Sox obtained him from Baltimore on June 3. He's a .267 lifetime hitter with 45 home runs in 656 career Major League games.
Manager Bruce Bochy said after the Giants' 5-4, 14-inning loss to the Dodgers that De Aza primarily will be used as a reserve but might start occasionally.
"It's good to have some help," Bochy said. "You probably saw some situations where we could have used him tonight."
Ysla, 23, was 3-6 with a 6.21 ERA in 33 games, including nine starts, for Class A Advanced San Jose.
The Giants designated Triple-A outfielder Ryan Lollis for assignment to clear 40-man roster room for De Aza.
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, follow him on Twitter at @sfgiantsbeat and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.