Looking to improve to 5-0 against the Colorado Rockies, Zimmermann will try to finish off an impressive month in the opener of this three-game series.
Harper showed he was fully healed from left thumb surgery by hitting three home runs in a rehab game at Double-A Harrisburg on Saturday. The outfielder did not play Sunday, and was reinstated from the 15-day disabled list Monday.
The 21-year-old Harper was hitting .289 with one home run and nine RBIs in 22 games before being placed on the DL on April 27.
Zimmermann (5-4, 2.95 ERA) would love to see Harper in the lineup with Washington having struggled offensively in his recent starts. The right-hander has gone 2-2 with a 1.18 ERA in five outings this month but hasn't earned a win in his last three, with the Nationals (43-38) backing him with one total run in those games.
Zimmermann gave up two runs and struck out nine in six innings in his latest outing Tuesday in Milwaukee, avoiding a loss when the Nationals forced extra innings in a 4-2 victory in 16.
Washington dropped three in a row following that win before sweeping a Saturday doubleheader against the Chicago Cubs. The surging Atlanta Braves, though, took over sole possession of the NL East lead with a 3-2 victory in Philadelphia on Sunday.
The Nationals were off Sunday, as Saturday's scheduled twinbill was a result of the city of Chicago wanting to avoid neighborhood traffic and congestion problems in connection with the city's annual Pride Parade.
"It's nice to win the last two," Williams said. "It's difficult to sweep a doubleheader anyway, so it's nice to end it on that note."
Zimmermann is 4-0 with a 1.42 ERA in six career starts against the Rockies, including 2-0 with a 0.42 mark in three matchups at home.
He'll face a Colorado team that has two wins in 13 games, though the Rockies (36-46) snapped a season-high six-game road losing streak with a 10-4 victory over Milwaukee on Sunday.
Troy Tulowitzki had three of Colorado's 15 hits and Corey Dickerson delivered a two-run double in the first inning. Dickerson, second on the team with a .343 average, departed in the eighth inning with a left hamstring cramp but said he would be fine.
Yohan Flande (0-0, 7.20) will make his second major league start. The left-hander gave up four runs, six hits and one walk in five innings of a 9-6 loss to St. Louis on Wednesday.
"I was relaxed, because I've been pitching for a long time," Flande told MLB's website through an interpreter. "I treated it like all those days in the minors."
Washington went 4-3 against Colorado in 2013.