To take Harper's place on the 25-man roster, outfielder Stephen Souza has been recalled from Triple-A Syracuse.
Harper sustained the injury sliding handfirst into third base on a triple Friday night. He went back in the field the following half-inning, but was later removed with what was first diagnosed as a jammed left thumb.
"It's sprained right now," Harper said. "I sprained it, they said. I'm just really trying to figure out in 15 days how I feel and not worry too much. We got a lot of off days in the next two weeks, that's huge. I'm really just trying to get a lot of treatment on it and see where I'm at."
A hand specialist examined Harper yesterday at Nationals Park and the 21-year-old was sent for an MRI on the thumb.
Harper will travel to Cleveland tomorrow to get a second opinion from Dr. Thomas Graham at the Cleveland Clinic.
"It sucks, definitely," Harper said. "It's part of baseball, you slide into the bag and sometimes it gets your fingers, that's just part of it. We'll see where I'm at at 15 and really try to get back as quick as I can."
Harper was hitting .289 with one home run and nine RBIs. He'll be replaced in the starting lineup today by Nate McLouth.
Souza, who made a strong impression during spring training and was one of the Nats' final roster cuts, hit .250 (1-for-4) in five games with Washington earlier this season. He was hitting .333 with two homers and 10 RBIs in 11 games with Syracuse.