"Well, I was concerned," Johnson said. "He wasn't running that great yesterday. He ran pretty good after some balls in the outfield, but I think he was just being on the safe side with it. If he can go, I need him in there."
Werth pinch-hit in the eighth inning of Monday's 3-2 loss to the Braves and struck out. His status for Tuesday is uncertain.
"He got a lot of treatment during the game and he was swinging down in the cage," Johnson said. "He said he was fine for a pinch-hit. I was gonna have to run for him."
Werth also was hit by a pair of pitches on Sunday, one in the ribs and one on the knuckles of his right hand. X-rays taken Sunday were negative, but Johnson said Werth was experiencing some swelling.
Werth was named National League Player of the Month on Monday after hitting .367 with seven home runs and 22 RBIs in July.
With Werth out, Johnson gave Steve Lombardozzi the start in left field. Scott Hairston was moved from left to right, with Bryce Harper in center.
Johnson said he wanted to give regular center fielder Denard Span a day off against left-hander Mike Minor while also taking the opportunity to get Hairston going. Span is hitting .151 against southpaws this season, while Hairston is 2-for-18 since the Nats acquired him from the Cubs on July 8, entering Monday.
Andrew Simon is a reporter for MLB.com. Follow him on Twitter @HitTheCutoff. This story was not subject to the approval of Major League Baseball or its clubs.