Valbuena, who was called up from Triple-A Iowa on Thursday, was in the starting lineup for the second consecutive day Friday in the opener of a three-game series against the Boston Red Sox. Stewart went on the disabled list with discomfort in his left wrist.
"There's his defense, a left-handed bat, he can drive the ball; I'll keep him in there," manager Dale Sveum said. "He was swinging the bat real well down there so you try to keep that left-handed bat in that lineup as much as you can."
Valbuena was batting .303 at Iowa, with eight home runs and 31 RBIs, but his 50 strikeouts were seven more than Anthony Rizzo.
The reality of the situation is that the left-handed hitting Valbuena probably will follow Stewart's schedule of playing against right-handed pitching while turning over third base to Joe Mather against left-handers.
The Cubs will face a lefty Saturday when Jon Lester starts for the Red Sox in the evening game at Wrigley Field.