Cardinals 1B Matt Adams sits against Nationals
Manager Mike Matheny said neither Adams nor the team's medical staff thinks the injury is serious.
Adams originally was listed as hitting fifth Monday night. He's batting .524 with three homers and eight RBIs in 24 at-bats.
With Adams out, Carlos Beltran moved into the starting lineup in right field, hitting cleanup. Allen Craig - slated to play right field in Matheny's original lineup - shifted to first base. Catcher Yadier Molina moved from fourth to fifth in the batting order.
Matheny said Beltran wasn't going to play Monday because of tightness in his lower back Sunday against Philadelphia.