The Nationals placed Harper on the DL Sunday and backdated the move to Saturday, but they are concerned that he could actually miss several weeks, according to the Washington Post.
"Doesn't look good," a source told the Post.
Harper, 21, was scheduled to see a hand specialist Monday to get more information about the injury's extent as it could be more than just a bruise.
Los Angeles Angels outfielder Josh Hamilton injured a thumb in similar fashion to Harper by sliding head-first into a base and is out six to eight weeks.
Harper was injured Friday night. Nationals manager Matt Williams said after the game that X-rays were negative. Harper missed Saturday's game. He was replaced in left field over the weekend by Kevin Frandsen but Nate McLouth may take over.
Harper is batting .289 this year but has just one home run in 83 at-bats. He was limited to 118 games in 2013 because of hip, knee and side injuries.