Nationals manager Matt Williams said prior to Saturday's game against San Diego that Harper was available "off the bench for sure." Williams said a hand specialist will look at Harper 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 did not appear in the Nationals' 4-0 victory over the Padres. He exited Friday's game in the fourth inning, one inning after he slid headfirst into third on a bases-loaded triple.
The Nationals did not disclose whether Harper, who said his thumb was swollen and tight, would undergo an MRI.
"I slid into the bag and I caught [the thumb] a little bit," Harper said. "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.
"We've got a great lineup and we're missing [Ramos] and [Zimmerman]," Harper said, according to the Washington Post. "I think where we're at right now swinging the bat, I think everybody needs to just keep it going, and we'll keep going every day.
"It [stinks]. But I feel like my swing is where it needs to be. I feel good, am staying on a lot of pitches and having good at-bats. Hopefully I come in [Saturday] and it feels pretty good."
Information from The Associated Press was used in this report.