Zimmerman hasn't played in a month due to a grade 3 hamstring strain. He was placed on the disabled list on July 23.
"When? I don't know, but sometime in September," Williams said to CSN Washington. "I don't think anything would happen until after [Sept. 1] because he still hasn't gone [through] the full gamut of tests. And then he would have to have some time to play."
The Nationals were a half-game ahead of the Atlanta Braves in the National League East when Zimmerman got hurt. Washington's lead over the Braves is now seven games going into Friday.
Zimmerman is hitting .282 with five home runs and 36 RBI in 53 games this season for Washington.
- Scooby Axson