Mercer was placed on the 10-day injured list Saturday with a left calf strain, Jesse Dougherty of The Washington Post reports.
The infielder presumably suffered the injury July 20, which was the last game he appeared in. Mercer had just been activated from the IL on July 16 after recovering from a quadriceps strain, but now he's set for another absence. It's unclear how long he'll be sidelined. Carter Kieboom was recalled from Triple-A Rochester to provide infield depth in Mercer's absence.