Soto apparently was hurt during Matt Holliday's at-bat in the Cardinals' first. The Cubs catcher came up limping after a foul ball by Holliday. Soto was lifted for pinch-hitter Koyie Hill in the Chicago first.
"He did the splits to block a ball in the first inning, and the splits were not good," Cubs manager Mike Quade said. "We need to be careful with this."
Soto has started 30 of 33 games and is batting .226 this season with three home runs and 12 RBIs.
Catcher Welington Castillo was pulled from the Triple-A Iowa lineup Tuesday and may be headed for Chicago for the Cubs' game Wednesday night as insurance. Soto is expected to be sidelined for a few days.