Bryce Harper will begin the most pivotal week of the Phillies' season as a designated hitter.

J.T. Realmuto, meanwhile, will spend one more game on the bench.

Harper informed manager Joe Girardi that the lower back stiffness that compelled him to leave Sunday’s game had eased enough to DH in Monday night’s series opener at Nationals Park in Washington. The Phillies are hoping Harper will be able to return to right field for at least one game in Tuesday’s doubleheader.

“I would anticipate that he’ll be able to play the outfield, but we’re just kind of day to day with just some back stiffness,” Girardi said. “We’ve logged a lot of games. They stand out in the outfield. There’s been some long innings. Hopefully [DHing] will kind of refresh him.”

Realmuto likely will catch one game Tuesday after missing 10 games with a strained left hip flexor, an injury that bothers him more when he runs than when he’s behind the plate, according to Girardi.

“I think our biggest concern is the burst out of the box,” Girardi said. “That’s how he hurt it originally, so I think that’s our biggest concern. Or if all of a sudden he had to make a burst on the bases because a guy misplayed a ball. We will remind him that, most of the time, he needs to have a governor on, like they do in NASCAR. You’ve got to be smart on how you run.

"J.T.'s an instinctual guy. J.T. plays as hard as anyone, so we took that into account. When we get him back, we’ve got to make sure that he’s pretty far along because of the way he plays.”

Indeed, the Phillies have been conservative with Realmuto, who is eligible for free agency after the season. They didn’t put him on the injured list because they believed he would return before the minimum 10-day term. If he catches Tuesday, he will return one day shy of that window.

Girardi said the Phillies estimated that Realmuto was at about 80% to 85% of his peak health on Sunday. With an additional day of rest Monday, they’re hoping he will return at about 90%.

“We feel comfortable doing that, but we also know the risk,” Girardi said. “If he comes back too early and reinjures it, then he’s out for a while. I think that you have to listen to the player, listen to the trainers, and then make a decision.”

It’s possible Realmuto will be available to pinch-hit Monday night. Girardi said Realmuto wasn’t an option in the last 10 games.

The Phillies also put struggling reliever Heath Hembree on the 10-day injured list with a sore right elbow and recalled outfielder Mickey Moniak from the Lehigh Valley site. Hembree, who had elbow issues last season with the Boston Red Sox, likely won’t be ready to pitch in the first round of the playoffs, if the Phillies qualify. He has allowed 13 runs on 17 hits, including seven home runs, and five walks for a 12.54 ERA in 11 appearances since being acquired in an Aug. 21 trade.

Additionally, right-hander Ramon Rosso was recalled from Lehigh Valley to take the roster spot vacated when Adonis Medina was optioned after making his major-league debut Sunday. Rosso is a possibility to start one of the games in Tuesday’s doubleheaders.

The Phillies will enter play Monday night with a half-game lead over Cincinnati, Milwaukee, and San Francisco for one of two wild-card berths in the National League. They’re also one game behind the Miami Marlins for second place in the NL East, a spot that comes with a playoff berth.