The move is retroactive to July 24. Drew has played just once in the past week, sitting out several days with dizziness.
"At first we thought it was the same bug that everybody else had, but theirs was like a two-day bug. This has been like a week now," manager Dusty Baker said. "So we decided we couldn't it play short anymore for the team and also try to find out what's wrong with him, so he can get back because he's a big part of our team."
The 33-year-old Drew, who signed with the Nationals as a free agent in January, is hitting .262 with seven home runs and 17 RBIs in 51 games this season. He is 6-for-20 (.300) with a triple, three homers and six RBIs as a pinch hitter.
On Saturday night against the San Diego Padres, Drew came off the bench after missing several days due to illness and hit a pinch-hit walk-off triple to give Washington a 3-2 victory.
The Nationals also recalled infielder Wilmer Difo from Double-A Harrisburg for his first major league assignment of the season.
Difo, 24, is hitting .255 with five home runs and 33 RBIs with Harrisburg. His 26 stolen bases rank fifth in the Eastern League. Of his 96 games played this season, 95 have been at shortstop for the switch-hitting Difo.