The team said the move is retroactive to Saturday. Harper jammed his thumb while sliding into third base Friday night and wasn't in the lineup Saturday against the San Diego Padres.
Outfielder Steven Souza Jr. was recalled from Triple-A Syracuse to take Harper's spot on the roster.
X-rays taken on Harper's thumb were negative. Manager Matt Williams said a hand specialist was to look at Harper's thumb on Saturday.
"The early tests today are pretty good," Williams said. "The X-rays are negative. There's some swelling there. We just want to make sure we knock it out."
Harper exited Friday's game in the fourth inning, one inning after he slid headfirst into third on a bases-loaded triple.
"I slid into the bag and I caught [the thumb] a little bit," Harper said Friday. "Went into the dugout, swung a couple in the cage, felt fine. Went out to the outfield, it swelled up a little bit."
The Nationals owned a 6-0 lead when Harper was removed from the game.
"I thought we were in a good spot, and [Williams] did too," Harper said. "So I got in and got some treatment on it."
Kevin Frandsen took Harper's place in left field on Saturday.
Harper went 2-for-2 and had had a career-high four RBIs on Friday night, giving him nine for the season as he lifted his batting average to .289. He has only one home run in 83 at-bats.
Information from The Associated Press was used in this report.