MASN television replays showed Harper holding his left wrist and flexing his left hand after the play. He stayed in the game for the fourth inning, but was replaced by Nate McLouth in left field to begin the fifth inning.
Harper left the game after going 2-for-2 with a single, a triple, one run scored and a career-high four RBIs.
The Nationals were leading 6-0 when Harper departed the game.
Update: Nationals manager Matt Williams said after the game that Harper sustained a jammed left thumb on his slide into third base. X-rays were negative and he will be re-evaluated Saturday morning.