Williams said Doug Fister, who has started in all but three of his 170 career games,will go to the bullpen.
"It's a difficult task when you've been a starter for so long, but [Doug's] willing to do whatever he can to help us win," Williams said.
Strasburg, 27, was placed on the disabled list July 5 because of a strained left oblique. He missed four weeks earlier in the season because of tightness in his neck.
In his latest minor league rehabilitation assignment for Triple-A Syracuse on Monday, Strasburg struck out 11 in 5 innings without allowing a walk.
"I feel good," he said after Monday's effort. "Stuff's there, so it's just trying to keep doing the same things and keep working at all the stuff I've been doing. I'm going in the right direction."
Strasburg is 5-5 with a 5.16 ERA in 13 starts this season for Washington.
Information from The Associated Press was used in this report.
Minute: Strasburg returning to the mound
The Baseball Tonight crew discusses Stephen Strasburg's return and how his performance is critical to the Nationals' playoff push.