Mercer (calf) will begin a rehab assignment at Double-A Harrisburg on Wednesday, Mark Zuckerman of MASNSports.com reports.
Mercer has been sidelined since July 24 due to a left calf strain, but he'll play second base and lead off in his return to game action Wednesday. The 35-year-old will likely require several rehab appearances following his lengthy absence, but he appears to be on track to rejoin the Nationals prior to the end of the regular season.