LaRoche had been out since May 10 with a strained right quadriceps before rejoining the lineup Sunday. He is hitting .319 with five home runs and 21 RBIs in 32 games.
The Nationals have hit just .194 during a season-worst four-game losing streak that has dropped them under .500 for the first time this season at 24-25.
Washington optioned rookie infielder Zack Walters to Triple-A Syracuse after Saturday's 3-2 loss to the Pirates to clear a roster spot for LaRoche. Walters hit .182 with three homers in 27 games.