Harper, 20, missed over a month due to bursitis in his left knee, only returning to the lineup on July 1. In five games since, Harper has struggled. He hit a home run in his first at-bat after being activated, but has since gone 0-18 with seven strikeouts.
Johnson believes Harper's issues are both mental and physical and that the outfielder is still working off some rust after a long layoff.
Harper suffered the initial knee injury after crashing into the outfield wall April 30 in an attempt to rein in a home run. He then exacerbated the injury by again hitting the wall hard on May 13 in Los Angeles.
Roger Bernadina will likely start in left field for Washington while Harper sits.