Strasburg was placed on the 15-day DL with neck stiffness, an injury that forced him to leave Friday's start after only 16 pitches. Harper was in Washington's original lineup Saturday but was still experiencing back soreness after being hit by a pitch Friday.
"I tried to get loose, but when I went on the field, it didn't feel very good," Harper said after the team's 8-5 loss. "It's more of a bone bruise. When I swing and I turn, it doesn't feel very good. I don't want to switch anything up. I don't want to try to do something else that I haven't been doing."
Strasburg, who returned to Washington, D.C., to be examined by team doctors, has struggled in May and failed to reach the fourth inning in four of his past five starts.
The former No. 1 draft pick recorded his shortest outing of the season Friday, when he began experiencing tightness in his neck while sitting in the dugout during the top of the second inning.
"We've got to get [to] the bottom of what's going on," Nationals manager Matt Williams said Saturday. "It's perplexing because it's not anything you can point to. So we'll get this thing worked out, and hopefully he'll be back in 15 days."
Williams hasn't decided who will take Strasburg's spot in the rotation. Right-hander Taylor Hill was called up from Triple-A Syracuse to shore up the bullpen for a few days as a long reliever. Hill was 3-2 with a 4.35 ERA in nine starts at Syracuse.
The red-hot Harper is tied for the major league lead with 18 home runs, and he leads the majors with 43 RBIs. He set a club record with 13 homers in May.
He was unhappy when Cincinnati's Tony Cingrani hit him in the back with his first pitch in the seventh inning Friday. He stared at the left-hander while taking his time going down the line.
Reds first baseman Joey Votto had an animated chat with Harper when he reached first base.
"What are you going to do?" Cingrani said. "He should've jogged. Be a baseball player. Sorry I hit you. Run."
Strasburg is 3-5 with a 6.55 ERA in 10 starts. The National League East leaders also are missing starter Doug Fister, who went on the disabled list May 15 with tightness in his forearm. The Nationals learned this week that outfielder Jayson Werth is out indefinitely as well, with two fractures in his left wrist, which was hit by a pitch.
The 26-year-old Strasburg hurt his left ankle during a conditioning drill in spring training. The Nationals want to figure out whether the problems are caused by Strasburg's changing his delivery to compensate for the ankle problem.
"Something's causing him discomfort in his upper back," Williams said. "The fact that it switched sides wouldn't point to anything structural. We've got to get it figured out.
"He's never had an issue with his arm. It hasn't been his elbow or his shoulder or anything like that. It's something to do with his back and his alignment. It could be from his ankle [injury] and the adjustment he's made from where he's landing and having to throw across his body more. We've looked at all of that."
The Associated Press contributed to this report.