Bryce Harper reacts on the field after being injured in the sixth inning against the Tampa Bay Rays at Nationals Park. (Rob Carr/Getty Images)

Bryce Harper rolled on his back on the damp outfield grass Thursday night at Nationals Park, grabbing his left leg in the sixth inning, grimacing in pain. Shortstop Ian Desmond was the first to rush out to right field. Athletic trainer Lee Kuntz, Manager Matt Williams and center fielder Denard Span were not far behind.

On the night Doug Fister returned from injury and the Nationals fell to the Tampa Bay Rays, 5-3, the sight of their best player, one of the top offensive forces in the National League, writhing in the outfield was difficult for an already banged-up team to watch. After a few minutes, Harper limped off the field under his own power, and after the game, Williams said the injury was a mild left hamstring strain.

Williams said Harper would undergo further evaluation Friday. Harper said he felt fortunate the initial diagnosis wasn’t as severe as the injury first appeared.

“With the way it felt, I’ve had problems with my left knee before, had surgery on it, so you know it was definitely really scary,” Harper said. “It didn’t feel all that great when I was laying there. So pretty scary, but being able to talk to the doc a little bit, see what he thought, see how it felt. Very happy with what it feels like.”

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For a team that has already experienced extended absences from more than one-third of its projected opening day lineup, any substantial time missed by Harper is no small cause for concern. He is batting .344 with 22 home runs and 53 RBI — among the leaders in the National League. Given that power sources Ryan Zimmerman and Jayson Werth are on the disabled list, Harper’s presence in the lineup is crucial.

“It’s time to step,” said Williams, visibly unhappy after the game. “It’s time. It’s time to step up. Everybody.”

Harper’s injury occurred on what seemed like an innocuous play. With two runners on and one out, Asdrubal Cabrera singled to right field. Harper charged the ball, scooped it with his glove and began his windup to throw home. But Harper’s back foot, his right one, slipped on grass still slick from the mid-game rain shower that caused a 17-minute delay. The slip caused Harper to land awkwardly on his left knee, which bent under his own weight.

“I came up throwing, and it was really slippery out there from the rain,” Harper said. “My right knee gave out, and then I put all my pressure on the left knee, got my hamstring — lower hamstring into the knee — a little bit. But it didn’t feel very good. Was in pretty good pain. Was a little scared at the same time because I could feel a little more on the lower half of it. But see how I am tomorrow and go from there.”

Harper said he had not undergone an MRI exam.

“It’s not to that point,” he said. “If it still feels pretty terrible [Friday] or the next day, we’ll take that, but we’re just going to see how it feels.”

Harper’s throw went wild, landing in the Rays’ dugout, allowing two runners to score and giving them the decisive runs. Harper tumbled to the ground. When Kuntz reached Harper, he sat him up and talked to him. Kuntz straightened Harper’s knee out and examined it.

After a few minutes, Harper stood up and walked off the field. He walked down the dugout steps by himself, and teammates smacked his back as he disappeared into the clubhouse with Kuntz.

Harper has started all but two of the Nationals’ 67 games. His 54 walks and 1.197 on-base-plus-slugging percentage are the best in baseball. He was playing well in right field, too. By some advanced metrics, he was the best overall player in baseball through the first 40 percent of the season.

“We’ve got to be ready,” third baseman Yunel Escobar said. “We have two big players on the DL: Werth and Zimmerman. Span has been nicked up. If Harper is hurt, what are we going to do? He’s in his moment right now.”

The Nationals had a 3-1 lead entering the inning behind Fister, making his first start since an extended stint on the disabled list with a muscle strain in his right forearm. Although his velocity was still around 86 mph, the same as it was earlier this season, Fister’s command was improved, and his diving fastballs produced groundballs. The Nationals staked him an early lead against tough Rays starter Chris Archer thanks to an RBI single by Harper and an RBI groundout by Danny Espinosa in the first inning.

The Nationals took a 3-1 lead in the second inning on an RBI single by Escobar, who had five hits in a game for the third time this season. Fister cruised through the middle innings until he slowed in the sixth inning. He gave up a leadoff opposite field solo home run to Joey Butler that trimmed the Rays’ deficit to one run. He then gave up back-to-back singles to Evan Longoria and Logan Forsythe.

With two on and no outs, Williams left Fister in the game. Fister got Steven Souza Jr. to groundout for the first out. Then came Cabrera’s single to right field that led to Harper’s frightful play. Harper was charged with an error, one of the Nationals’ three; Fister was pulled from the game; and the Rays took the lead. The Nationals’ attention then turned to Harper.

“There’s no question he’s been our best player. He’s been the first-half MVP for us, so hopefully he’s not out too long,” Span said. “We definitely need him in the lineup with all the injuries we already have. We have a lot of key players out. Hopefully he’s not out too long.”