Washington (55-44) comes off a 6-4 loss at Colorado on Wednesday but has won 14 of 20. Jayson Werth has batted .357 with 11 doubles, six homers and 22 RBIs in that stretch, and while he sat out the finale against the Rockies due to a sore knee, manager Matt Williams told the league's official website "it's nothing big."
Denard Span is hitting .355 in that span while Anthony Rendon has scored 22 runs.
The Nationals weren't entirely discouraged by Wednesday's defeat, as they scored two runs with two outs in the ninth before Ian Desmond struck out with the bases loaded. It was their first game since Zimmerman was placed on the disabled list with a strained right hamstring.
Washington is 34-19 with Zimmerman in the lineup. The third baseman missed 44 games earlier this season due to a broken thumb.
"We did a good job battling back right there," said Desmond, who owns a .383 average with 13 RBIs over his last 15 contests. "We kept on grinding away. I'll take us in that situation a lot of times."
The Reds (51-50) are still looking for their first post-All Star break victory after a winless six-game trip. Cincinnati has averaged 2.0 runs during the skid, and RBI leader Todd Frazier is 4 for 23, though he's batted .389 over his last 10 against Washington with 11 RBIs.
The Reds haven't lost seven in a row since an eight-game slide July 28-Aug. 4, 2009.
"I think that our guys are capable of turning it around," manager Bryan Price said.
Simon (12-4, 2.74 ERA) had won six straight decisions before a 7-1 road defeat to the New York Yankees on Saturday. He allowed four runs - two earned - over five innings, but has posted a 2.09 ERA in his last seven outings.
He earned the victory in a 2-1 win over Washington on May 21, throwing seven innings and allowing a leadoff homer by Span.
Tanner Roark (9-6, 2.91) gave up an earned run over six innings in opposing Simon.
Washington has developed a habit of scoring early for Roark. The right-hander limited Milwaukee to one run over seven innings of an 8-3 victory Saturday, getting backed with five runs in the first inning. The Nationals have scored in the first in seven of his last eight outings, totaling 18 runs.
"It's a lot weight off my shoulders whenever we get five runs in the first," said Roark, 6-3 with a 2.31 ERA in his last nine starts.
Span has gone 15 for 35 (.429) in his last seven matchups with Cincinnati and was 9 for 16 as Washington dropped two of three to the Reds from May 19-21.