"I wish it had gone a little better," Stauffer said of his night.
It might have, although the Padres -- who scored more than three runs in a game for the first time since May 4 -- gave the lead back for good when rookie relief pitcher Miles Mikolas allowed two runs in the sixth inning.
Reliever Luke Gregerson then allowed two home runs in the eighth inning for the first time in his career as the Padres fell to 12-24 this season.
"You hate losing games when you swing the bat well," said Padres third baseman Chase Headley, who had two hits, knocked in a run, and bowled over rookie catcher Sandy Leon in the third inning.
"That's not a shot at our pitching ... our pitching has been great."
After having four different starting pitchers on the disabled list at one point -- Stauffer, Dustin Moseley, Joe Wieland and Cory Luebke -- the Padres certainly felt good about their chances with Stauffer taking the ball Monday. He was in line to start Opening Day before his elbow issue flared up late in camp in Spring Training.
"It was nice to get out there again," Stauffer said.
Against the Nationals, he wavered at times, allowing an unearned run in the first inning and then three in the third inning -- including the solo home run to Harper, a ball that sailed beyond the fence in center field and onto the grass berm, some 420 or so feet from home plate.
"He's a talented, young player, an exciting, young player. He plays hard, runs the bases hard, and looks like a guy who takes an aggressive swing," Padres manager Bud Black said of Harper. "At 19 ... that says something."
Despite the Harper home run, the Padres were in a good spot after the fourth inning when they scored three runs, two on an Orlando Hudson single and one on bunt by Stauffer. Headley had a double in the inning and scored on Hudson's single, but not after colliding with Leon at the plate.
"Coming in, you intend to slide until there's nowhere to go," Headley said. "He really left me nowhere to go. You hope that he's going to be OK. It's a bummer to have that happen in your debut."
The Nationals announced during the game that Leon had a high ankle sprain. He's headed to the 15-day disabled list.
"It was a clean play. He was waiting on the throw, and the runner beat the throw," Nationals manager Davey Johnson said. "He is such an outstanding young man. It was his first big league game, and [he was] all pumped up. He is hurt his first game, that's tough."
As for Stauffer, he struck out five and walked three. He had 91 pitches, exceeding the 78 pitches that he threw in his final Minor League rehabilitation tune-up last week.
"Unfortunately, I put us in a little bit of a hole," Stauffer said. "It was a battle from the get-go. Obviously, there are some pitches I'd like to have back."
Stauffer finished strong, striking out Danny Espinosa on a slider after runners reached second and third base with one out in the fifth.
"He made some good pitches all night," Black said. "Overall, I thought his stuff was fine. It's a good one to build off."
Having taken the lead in the fifth inning and with Stauffer finished, Black went to his bullpen for Mikolas, who was making his fifth big league appearance, in the sixth. Mikolas, who had a reputation as a strike-thrower in the Minor Leagues (42 walks in 219 innings), immediately walked Rick Ankiel to set up a big inning.
Before the inning got out of hand, Black went to the mound to encourage Mikolas, who was pitching for Class A Lake Elsinore a year ago at this time.
"Turn it loose," Black told Mikolas, who has walked seven in 3 1/3 innings.
A well-placed hit-and-run by Jose Flores sent Ankiel to third base. He didn't stay there long, as just two batters later, Ian Desmond got his third hit of the game, a two-run double, to put the Nationals on top for good.