Fister missed a little more than a month of action because of a flexor strain. Williams didn't say how many pitches Fister would throw in the game, but he would not throw 110 pitches.
Fister rejoined the Nationals after two rehab starts and had a bullpen session Monday and didn't experience any problems.
"He is ready to go. Let's see where we can take him to," Williams said.
With Fister expected to pitch Thursday and right-hander Stephen Strasburg not too far behind, the Nationals have to make a decision about their rotation. Right-hander Tanner Roark started Tuesday against the Rays and could go back to the bullpen, while right-hander Joe Ross could be sent down to the Minor Leagues. Roark and Ross held their own while they were in the rotation.
"We are going to see how it goes," Williams said. "[Tuesday] is a really good start for him. With Doug being ready and Stras coming back, I'll see where we stand with all of it. Hopefully, today [Roark] makes it difficult for us to make a decision.
"[Ross] has pitched well. He has given us an opportunity both times he has been out there. He pitched really well in Milwaukee. It's always a difficult decision. What we like is that he is not afraid, he puts the ball over the plate. He's a good athlete, he hits, he runs well. He does a lot of things on the baseball field that allows him to help himself win games."