Nationals manager Matt Williams said the plan was for Harper to play three innings in left field and get a couple at-bats. He was expected to play all three outfield positions while on rehab. He could return around July 1.
The 2012 NL Rookie of the Year was hitting .289 with a homer and nine RBIs before injuring his thumb on a headfirst slide into third base. He was put on the disabled list on April 27 and had ligament surgery two days later.
The Nationals also moved catcher Wilson Ramos' rehab assignment for a strained right hamstring to Potomac on Monday.
Washington was in Milwaukee to play the Brewers.