Harper exited Washington's 11-1 victory over San Diego in the fourth inning, one inning after he slid headfirst into third base on a bases-loaded triple.
Nationals manager Matt Williams said X-rays on the thumb were negative but acknowledged that Harper would be evaluated again Saturday.
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," said Harper, who had a career-high four RBIs. "So I got in and got some treatment on it."
"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.