ESPN.com says the two-time All-Star will have the operation Tuesday.
Washington spokesman John Dever declined to comment. The team was off Monday, and plays Tuesday night at Houston.
The Nationals put Harper on the 15-day disabled list Sunday, saying he had a sprained left thumb.
Harper was hurt Friday night on a headfirst slide into third base on a triple against San Diego. He didn't leave the game until after the next inning.
"I slid into the bag and I caught it a little bit," he said about his thumb after that game. "Went into the dugout, swung a couple in the cage, felt fine. Went out to the outfield, it swelled up a little bit. ... So I got in and got some treatment on it."
Harper is hitting .289 with one home run and nine RBIs.
The injury came less than a week after Harper was yanked from a game for what manager Matt Williams termed "lack of hustle" after heading toward the dugout instead of running out a groundout.
For the 21-year-old Harper, the 2012 NL Rookie of the Year, this will be the second prolonged injury absence of his brief career.
Last season, he missed 31 games because of bursitis in his left knee after crashing into outfield walls twice. He wound up having an offseason operation on the knee.
He said this winter that he hadn't been 100 percent healthy even before running into walls while playing defense in games on April 30 and May 13 a year ago. Harper finished last season with a .274 batting average, 20 homers and 58 RBIs.
Nate McLouth is expected to get extra playing time during Harper's absence. McLouth signed a $10.75 million, two-year contract with the Nationals as a free agent this offseason.
McLouth is hitting .118 — four hits in 34 at-bats — with one RBI in 2014. He homered Sunday against the Padres while starting in Harper's place.
Harper's injury is only the latest in a string to key players for the Nationals, and he joins two other members of the everyday lineup who also happen to be middle-of-the-order hitters — third baseman Ryan Zimmerman and catcher Wilson Ramos — along with a key offseason addition to their starting rotation — right-hander Doug Fister — on the disabled list.
Ramos broke a bone in his left wrist on opening day, and Zimmerman broke his right thumb on April 12 while diving into a bag, similar to Harper. Fister has been sidelined since spring training with a back muscle injury.
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