Gonzalez made his second rehab start for Single-A Potomac on Thursday as he works his way back from left shoulder inflammation, which has shelved him since May 18.
Williams indicated the shoulder is doing well enough for Gonzalez to return.
"Yeah, barring some kind of issue with his bullpen," Williams said. "But everything was good last night, felt good, no pain, no discomfort in the shoulder. He got to 79 pitches and feels good about it. So given the fact that he's slated to come back, we decided to make a move and get Xavier (Cedeno) here. So we'll make a decision on Gio depending on how these three games go and where we slot him in at. With the off-day, we've got some flexibility there."
Because of Monday's off-day, Gonzalez could start for Washington on either Tuesday or Wednesday. Knowing that the southpaw is on the cusp of being activated, the Nats optioned right-hander Blake Treinen to Triple-A Syracuse and added the left-handed Cedeno to the bullpen for the weekend.
Treinen is 0-3 with a 2.08 ERA in eight appearances (four starts) for the Nats this season.
"I think Blake did a really nice job - pitched well and competed and all of that. But of course we want Gio back," Williams said. "He's won a lot of games in the big leagues, so we want him to be a part of the rotation here. So we're glad to have him back in it healthy and feeling good, so that's all a good sign."
As for Gonzalez, he allowed one run on two hits while striking out seven and walking four in his rehab start for Potomac on Thursday. The brief outing and high walks total had to do with Gonzalez working on specifics as he prepares to rejoin the rotation.
"He threw a lot of breaking balls working on his breaking-ball command," Williams said. "He had a bunch of strikeouts, so that will make the pitch total go up a little bit. But he was working on changeup, working on breaking-ball command and going through his start, just making sure that he got that down and got a feel for that. Velocity was good, health was good, so he should be good to go."
In the meantime, the Nats open a three-game series at St. Louis, facing a Cardinals team that caused all kinds of trouble for Washington last season.
The Cards won all six meetings against the Nats in 2013. Washington is off to a much-improved start against St. Louis this year, having split a four-game series against the Cardinals at Nats Park.
Williams doesn't believe last year's problems against St. Louis will linger with the Nats.
"Every game's different. So one day can be a struggle. The next day can be everything goes right for you," he said. "So it's human nature to think about those things, but I think you have to put it out of your mind as much as you can. Again, I don't have the history that most folks have here. I can tell you this. That team over there, they do a really nice job with guys in scoring position. They create those opportunities and they come through.
"They've got good, professional hitters, they generally play good defense and they pitch solid. So that's probably a recipe for a lot of teams not playing well against them. We've seen that over the course of the last couple years the way they've played. That's all in the past though. We have to concentrate on tonight. We've got Jordan (Zimmermann) going for us tonight. He pitched really well his last time out and hopefully that equates to a win for us."