Valbuena, who primarily played at second base with Iowa, made his second start on Friday.
"[We like] his defense and left-handed bat -- he can drive the ball," Cubs manager Dale Sveum said. "We'll keep him in there. He was swinging the bat well down there. You try to keep that left-handed bat in the lineup as much as you can."
Valbuena, who was batting .303 at Iowa, said third is more difficult than second, especially when a right-handed hitter is in the batter's box.
"It's harder," Valbuena said. "You have to be ready every time."