Fister was placed on the 15-day disabled list Friday after experiencing right forearm tightness. An MRI revealed the strain.
The right-hander will rest and rehab the injury, according to Williams, who noted tests on Fister's elbow came up clean. There is no timetable for his return.
"It depends on how long it takes to calm down," Williams told reporters. "Of course, any time you've got a strained muscle you're going to have to shut down for a while. Everybody is different in that regard. We'll make sure that it's calm when he starts to throw again."
Fister is 2-2 with a 4.31 ERA for the Nationals. He was rocked in his last start, allowing seven runs in just two innings against the San Diego Padres on Thursday.
Any lengthy absence from the rotation would be a tough blow for Washington, which saw Fister go 16-6 with a 2.41 ERA last season, his first with the team.