Zimmerman, who hasn't played since July 22 because of a severe hamstring strain, underwent another MRI on Monday and the medical staff liked what it saw.
"The doctors looked at it and it's healing up really nicely, so he can start increasing his strength stuff," Williams said. "So he's jogging now and he's continuing to throw a little bit here and there every other day or so. No hitting yet. The first test is him running full speed, and then it's bases, certainly BP and getting his swing back and all that. But he's making the progress that he should make.
"So strength will increase now that he can start pushing it a little bit with that. And once he feels good about that, then he can get out and start running and doing some sprint work, all the while being mindful that we don't want to push it too hard because if you lose him, you lose him for good at this point."
Williams didn't provide a new timetable for Zimmerman's return, but has said in the past that the club hopes to have the veteran back sometime in September. The improved MRI was good news for the time being.
"It's positive given the time he's been off and the stuff that he's been doing. Very positive," Williams said. "Again, it's by no means ready to just go sprint right now. But healing up good and now he can increase that strength work, which will really get him over the hump."
Zimmerman has not progressed beyond light throwing and straightforward jogging with no lateral movement, changes in direction or spinning. He also hasn't begun dry swings. He has to restore strength in the hamstring first.
As for tonight, Jayson Werth is getting a game off his feet as he's serving as the Nationals' designated hitter at Safeco Field.
Werth has had success in eight career games as a DH, going 7-for-22 with a .318/.423/.636 slash line, two homers, six RBIs and six runs scored.
Williams said part of the reason Werth is filling the role is to rest his sprained ankle.
"Yeah, as much as possible," Williams said. "And we want to get Nate (Schierholtz) in the game, too. He hasn't had a start yet. He's a good defender out there, he's certainly comfortable playing out there. So it gives us an opportunity to get him in the game and take a little bit of pressure off Jayson, too, with his ankle. So it's a logical move for us."
Williams doesn't anticipate it being too much of an adjustment for Werth, who has been a DH before.
"It's difficult for National League guys because they're used to playing in the game. They're used to that rhythm of the game," the manager said. "So you have to keep yourself busy between at-bats. So it's easy just to sit down and watch the game, but you have to be mindful of making sure that you're loose and keep yourself busy because you don't get a chance to run out to the field. He'll be good. He's done it before."
Williams wouldn't commit to Werth being the DH for the whole series, in part because there was some uncertainty about the Mariners' pitching order for Saturday and Sunday. And Seattle did announce a change before tonight's game, inserting rookie left-hander Roenis Elias on Saturday, moving former Nats farmhand Chris Young from Saturday to Sunday and pushing right-hander Hisashi Iwakuma out of the series altogether.
Williams was also asked about September call-ups. While he wouldn't disclose any names or how many players will be added, he said those joining the club will meet the team in Los Angeles on Monday and "there will be a number of guys" being brought up in September.