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.
X-rays were taken on both his ankle and elbow. Both were negative.
"It's just swollen. It's tight, sore. Other than that, it's not too, too bad," Howard said.
He wasn't savoring his initial sprain, though.
"It was my first time. I wouldn't recommend it for anybody," Howard deadpanned.
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.
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.
Information from The Associated Press was used in this report.