Manager Matt Willliams told reporters after the game that Harper has a mild left hamstring strain.
The game had been delayed by rain earlier and Harper's right foot slipped on the wet grass as he attempted to make a throw. The ball sailed into the third-base dugout, allowing two runs to score.
Harper immediately began clutching his left knee and grimacing in pain. He was able to leave the field under his own power and was replaced by Clint Robinson.
Harper leads the majors in slugging (.726) and OPS (1.204). He has 22 home runs, tied for the second most in the majors, and leads the majors with 9.8 at-bats per home run.
Harper missed last Sunday's game after he was hit in the left knee by a pitch.
The Rays went on to win Thursday's game 5–3, giving them a two-game home sweep of the Nationals and three wins in four consecutive games against Washington.
- Dan Gartland