With Harper batting a season-low .246 and struggling since mid-April, manager Dusty Baker said he wanted the 23-year-old to take a "mental day off."
That includes sitting out against the Mets, at least early, as well as any work in the batting cage.
"I told him to do nothing, just concentrate and watch the game," Baker said. "You can straighten yourself out by watching others and by what they're doing or not doing."
Baker said Harper "was cool with" the decision.
Harper is 0 for 9 over his last three games, and he hasn't homered since he hit his 11th of the season on May 13. Since April 18, the reigning National League MVP has hit .202 (20 for 99) over 34 games. He also drew 40 walks over that stretch, with his on-base percentage now sitting at .432.
Harper began the season where he left off. By the 11th game, he was hitting .359, and by his 18th, he had homered nine times.
In 2015, Harper led the NL with 42 homers and 118 RBIs, and his .330 batting average trailed only the Miami Marlins' Dee Gordon (.333). Since breaking into the majors at age 19 in 2012, he's hit at least .270 and played in at least 100 games each of his first four seasons.
"This is probably the worst period Bryce has gone through," Baker said. "Every time you go through a bad period or a bad stretch, it's the worst time in your whole life, until the next time."