Valbuena got the start at third base for Thursday's game against Justin Verlander and the Detroit Tigers. The Cubs claimed him off waivers from the Toronto Blue Jays in April, and he hit .303 with eight homers and 31 RBIs in 58 games with Iowa.
Castillo was placed on the DL on May 22 with a sprained right knee.
Stewart was scratched from Wednesday's game. He is batting .201 with five homers and 17 RBIs in 55 games in his first year with Chicago.