The lineup has not yet been posted, likely because team trainers are evaluating one or both of the Nats' ailing outfielders.
Harper left last night's game following a check swing in a sixth-inning at-bat. He was diagnosed with a bad bruise on his left side.
The Nationals' leader in nearly every offensive category had originally injured the side when trying to make a leaping catch against the wall in right field on Tuesday night.
Nats manager Davey Johnson told 106.7 The Fan earlier today that Harper underwent X-rays, but they came back negative. As the Nats had originally thought, Harper does not have any broken ribs, but given Johnson's desire to take things slowly with his injured players, it would be a surprise to see Harper in tonight's lineup.
Werth, meanwhile, has a shot to play tonight after missing the last two games with a sore left ankle and hamstring cramps. X-rays taken on Werth's ankle before last night's game came back negative.
If Harper and Werth are both out, Johnson will be working with a pretty short bench tonight. In such a scenario, Tyler Moore would likely start in left with Roger Bernadina in right.
The Nats will send Dan Haren to the mound tonight as they look for a split of this four-game series.
Haren had his best start as a National his last time out, allowing two runs on six hits over six innings in a win over the Reds. The veteran right-hander comes into this outing with a 2-3 record and a 6.29 ERA.
Kris Medlen (1-3, 3.26 ERA in five starts) goes for the Braves tonight. Medlen has walked nine and struck out 18 in 30 1/3 innings. He's posted a 1.50 ERA since the 2012 All-Star break, best in the majors.
In two starts against the Nats last season, Medlen allowed one earned run in 14 innings, striking out 20 and walking two.
Harper went 2-for-6 with a homer and a walk off Medlen last season, but that obviously won't matter if he is held out tonight.
Ian Desmond is 2-for-9 with two doubles off Medlen. LaRoche has a double and three hits in 12 at-bats against the Braves right-hander.
For the Braves
RF Jordan Schafer
3B Chris Johnson
LF Justin Upton
1B Freddie Freeman
C Evan Gattis
2B Dan Uggla
CF B.J. Upton
SS Andrelton Simmons
RHP Kris Medlen