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 was sliding headfirst, he jammed the thumb into the base. Harper 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.
"It was a sprain like they said, and they are really trying figure out in 15 days how I feel," Harper said. "I'm not worried too much. We have a lot of off-days in the next two weeks, so that is huge. I'll try to get a lot of treatment on it. We'll see where I'm at."
Manager Matt Williams said Harper did nothing wrong in the way he slid into the base. It was simply bad luck.
"It's happens all the time," Williams said. "Guys slid in head first 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."
Harper will mostly use ice to treat the thumb injury in the short term and will go to Cleveland this week to see a hand specialist for a second opinion. According to Williams, Harper has a history of thumb problems dating back to high school.
"There's no break, there is no fracture. The MRI shows sprain. That means many things," Williams said. "You get guys that play with it all the time. So he is going to get a second opinion. We put him on the DL for now until he gets that second opinion, get the swelling out, evaluate how he is and see where he is at."
With Harper out of the lineup, McLouth, Kevin Frandsen and Tyler Moore will see time in left field, with McLouth getting most of the time against right-handed pitching.
As for Souza, he is with the Nationals for the second time this season. He was with the team April 12-18 and was 1-for-4. He was also 11-for-33 (.333) with two home runs and 10 RBIs for Syracuse.
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.