Howard was injured as he scrambled back to second base after a throw from Nationals right fielder Michael Morse.
Howard had to be helped off the field by manager Charlie Manuel and a team trainer.
Howard, who had sat out his first game of the season on Saturday night, is in an 0 for 14 slump. He reached base when he was hit by a pitch on his right elbow from Washington starter John Lannan. X-rays were taken on both his ankle and elbow. Both were negative.
The Phillies have two starters - second baseman Chase Utley (sprained right thumb) and center fielder Shane Victorino (abdominal strain) on the disabled list. Left fielder Raul Ibanez was a late scratch from Sunday's game with a sore left wrist. Ibanez hurt the wrist diving for a ball in Saturday night's game.
Howard is batting .292 with 23 home runs and 81 RBIs.
