Harper went 0-for-4 with a sacrifice fly and a strikeout on Friday, making him 0-for-18 since he hit a home run in his first at-bat off the disabled list on Monday against the Brewers. He has three walks and seven strikeouts over that span.
Johnson believes Harper's issues are probably both mental and physical. The 20-year-old missed a little more than a month with bursitis in his left knee, then completed a four-game Minor League rehab stint before returning to the Nationals.
"He's just … coming back from the long layoff, I think hadn't had much break when he came back," Johnson said. "I just need to back off him a little bit."
Harper has registered some good at-bats over the past few games. On Tuesday, Brewers center fielder Carlos Gomez twice robbed him of hits with highlight-reel catches, including one at the warning track. Harper walked twice on Thursday, and in his second at-bat on Friday, he hit a fly ball to the warning track in center to drive in a run.
But in his final three at-bats on Friday, Harper saw a total of six pitches, including a strikeout. His average has dropped to a season-low .260.
Andrew Simon is a reporter for MLB.com. Follow him on Twitter @HitTheCutoff. This story was not subject to the approval of Major League Baseball or its clubs.