Zimmerman, who broke the thumb diving back to the bag as he was picked off second base against the Atlanta Braves on April 12, has missed 40 games.
Nationals manager Matt Williams said there is still not timetable for his return.
"We don't have a definite on that yet," Williams told reporters, according to the Washington Post. "But he's taken the next step, which is good. So he'll start swinging the bat in the cage, some tee work, some soft-toss work. See how he does with that and progress from there."
Zimmerman was hitting .364 with two home runs and six RBIs before the injury.