Eury Perez replaced Harper in the lineup for Washington's 8-0 win. Perez played center field, while Brett Carroll was switched from center to right.
Harper said he woke up Thursday morning with the tightness. He then went to Space Coast Stadium and exercised on the bike to loosen the calf, before doing some outfield drills. But the pain didn't go away.
"I never felt it before," Harper said. "That was the weird thing. I felt it this morning. I have no clue what's going on right here. I came in and tried to ride the bike and tried to loosen it up. I didn't feel it when I swung [the bat]. I felt it getting reads in the outfield."
Entering Thursday's action, Harper was 5-for-11 (.455) with five singles. His five hits led the Nationals. Manager Davey Johnson said he was going to give Harper every chance to be Washington's everyday right fielder.