"On the road to recovery," Harper wrote. "Everything went great and I'll be back soon!"
The original diagnosis was that Harper had a sprain and that he would be available for pinch-hitting duties, and Harper said he would use nothing but ice to reduce the swelling. However, he was placed on the 15-day disabled list on Sunday.
Harper hurt the thumb in the bottom of the third inning Friday against the Padres. With the bases loaded, Harper hit a three-run triple to right-center off left-hander Robbie Erlin. As Harper slid headfirst into third, he jammed the thumb into the base. He later scored on a single by Jose Lobaton.
Harper went 2-for-2 with four RBIs in the game before being replaced by Nate McLouth in the fifth inning.
Manager Matt Williams said Harper did nothing wrong in the way he slid into the base. It was simply bad luck, he said.
"It's happens all the time," Williams said. "Guys slide in headfirst all the time. Just so happens that he got his thumb in an awkward position. You can't question that. You can't question the way the kid plays the game. He plays the game hard."
This is Harper's second major injury since he made his big league debut in 2012. In May of last year, Harper ran into an outfield wall at Dodger Stadium and sustained a left knee injury that required offseason surgery.
With Harper out of the lineup until close to the All-Star break, McLouth, Kevin Frandsen and Tyler Moore are expected to see time in left field, with McLouth getting most of the playing time against right-handers.
The Nationals could try and trade for a player of Harper's caliber, but it seems doubtful they could add payroll. Earlier this month, principal owner Mark Lerner was asked if the club has the flexibility to spend more money, and Lerner said, "We're beyond topped out. Our payroll, as you know, has skyrocketed to about $140 million. I don't think we can go much farther with the revenue stream that we have."
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.