Bryce Harper stood in the Washington Nationals’ dugout as Nationals Park roared for him, with the unfamiliar and splendid feeling of being unsure of what he should do. Teammate Jayson Werth, 14 years his senior, slapped him on the back and let him know it was okay. Werth told Harper, “Get up there, kid.”

Harper bounded up the steps and pointed to the 19,434 fans. He was still breathing hard from his rapid trip around the bases and the scale of the moment. Harper had hit a home run over the center field fence, a first he had waited all of his 19 years for, a moment to celebrate during the Nationals’ 8-5 victory over the San Diego Padres.

“I’ve got no words for it,” Harper said. “Hopefully it’s one of many.”

Sandy Leon had waited, too, for Monday night to come. His family and friends watched on television from his home in Venezuela as he made his major league debut. He earned his way to the majors, at 23, for his skill behind the plate, for not allowing runs to score easily. So as Chase Headley bore down on him in the fourth inning, Leon blocked the plate. Moments later, Leon left his first big league start with a sprained ankle, his arms slung over the shoulders of a trainer and a coach.

“He’s such an outstanding young man,” Manager Davey Johnson said. “His first big league game, all pumped up. That’s tough.”

The two firsts, Harper’s joy and Leon’s deflation, defined a wild Nationals win. The Nationals jumped to a 4-1 lead, which left-hander Ross Detwiler could not hold. Long reliever Craig Stammen continued his invaluable season with two scoreless relief innings, holding the Padres until Ian Desmond’s two-RBI double high off the right-center wall in the sixth, part of a 3-for-5 game, gave the Nationals the lead.

Chad Tracy and Xavier Nady added home runs in the eighth to provide breathing room. The Nationals needed it. In the ninth, hours after Johnson affirmed his faith in closer Henry Rodriguez after a calamitous ninth inning Sunday, Rodriguez threw his second pitch to the backstop and walked the bases loaded with one out.

Sean Burnett replaced him and saved the day by inducing a 1-2-3 double play. “That’s the way you’d draw it up,” he said. The Nationals lined up and shook hands, and Johnson again professed his faith in Rodriguez.

“I still have a lot of confidence in him,” Johnson said. “I went up to him after the game, I said, ‘Henry, you’re my man.’ ”

Rather than celebrate another victory, the Nationals had to absorb another injury. Two days after Wilson Ramos tore his right anterior cruciate ligament, the Nationals placed Leon on the disabled list with a right high-ankle sprain and planned to call up Carlos Maldonado from Class AAA Syracuse. Leon became the 10th National to spend time on the DL this year.

The injury scrubbed away the festive atmosphere Harper’s home run provided. Harper came to the plate in the third inning for his 62nd major league plate appearance and his 54th at-bat. Harper had hit only one homer in 72 minor league plate appearances, but he had not thought about the drought.

“I’m just trying to get as many knocks as I can that day,” Harper said. “I’m not really that big of a big, big home run guy.”

He jumped ahead of Padres right-hander Tim Stauffer, 2-1. Stauffer fed Harper an 87-mph slider, and it broke over the middle of the plate, at his thighs. Harper stayed back and crushed it to dead center field.

“You don’t really know when you hit a ball like that,” Harper said. “It was a line drive that just kept on going.”

The ball landed and bounced on the grassy hill about 15 feet beyond the center field fence, to the left of the 402-foot mark on the wall. Harper became the youngest player to hit a homer since Adrian Beltre in 1998.

Harper had bolted out of the batter’s box, and once the ball cleared the fence he never slowed his gait. Harper ran around the bases in 18 seconds and kept sprinting all the way into the dugout.

“I always have since I can remember,” Harper said. “I don’t want to show up that pitcher. The only time I would do that [would be] if they were messing with my team.”

Johnson met him with a high five and his teammates followed suit, patting him on the head and the backside.

“I think we all knew it was a special moment,” Tracy said.

The ball remained on the hill until a Nationals clubbie walked out and grabbed it. Nationals relievers jokingly lamented to one another that the ball — and the ransom that would come with it — hadn’t landed in the bullpen. Later, it rested on a shelf in Harper’s locker. He planned to toss in a cardboard box with other balls, including the first homer he ever hit, at age 7.

The blast gave the Nationals a 4-1 lead, but the Padres immediately charged back. With one out in the fourth, with Headley on second base, Orlando Hudson ripped a single to center off Detwiler.

Headley sprinted around third, trying to score. Center fielder Rick Ankiel fired to second baseman Danny Espinosa, who made the relay throw home. Leon stood in front of the plate. Headley plowed into him as Espinosa’s throw skipped to the plate on one hop.

“It was a clean play,” Johnson said. “He was waiting on the throw, and the runner beat the throw.”

As Headley barreled into him, Leon’s right ankle rolled back, all of his weight on the joint. The ball bounded to the backstop. Headley walked away. Leon remained down in the dirt by home plate. Trainers tended to him and carried him off the field, Leon putting no weight on his right leg.

“I thought it was a clean play and I’m sorry it resulted the way it did,” Headley said. “When I was coming in, I was intending to slide. He had the plate totally blocked and the ball wasn’t there yet.”

Leon is a highly regarded defensive catcher. He found out he was making his major league debut at about 3:30 p.m., when Johnson told him. Back home in Venezuela, he knew his parents, family and friends would be watching. “I feel a little nervous,” Leon said before the game. “But I feel I’m ready to play. That’s why I’m here.”

A few hours later, Leon snapped on his shin guards, squatted behind home plate and began his career. He came off the field in a way neither he nor the Nationals could have imagined. He left the Nationals clubhouse with his foot in a large, black boot supported by a rolling wheel. Another cruel injury had added a sour note to a victory and another, happier first.