"Pretty much like yesterday, Adam was available to pinch-hit and we hope that's the case today," Riggleman said. "He's not right yet. We're going to keep him out of there for another day and see how he feels tomorrow."
The left-handed hitter blasted two home runs during Saturday's doubleheader in Colorado, but he was out of the starting lineup the next day, though he pinch-hit in the ninth inning and grounded out to end the game.
Riggleman had hoped Dunn would be ready to go on Monday.
"I basically made two lineups," he said. "When he came in I thought he would feel better and he didn't, so I made that change."
The manager then moved second baseman Adam Kennedy to first base and moved left fielder Josh Willingham from fifth in the lineup to fourth. Ian Desmond, who was not in the original lineup, was then inserted to play shortstop, with Christian Guzman moving from shortstop to second base.
Dunn, in his second season with the Nationals, is hitting .254 this season with 10 doubles, one triple, nine home runs, 19 RBIs and 21 runs scored. He has a .378 on-base percentage and a .554 slugging percentage.