WASHINGTON -- Roy Oswalt's first game as a Philadelphia Phillie left ample room for improvement.

Craig Stammen carried a shutout into the seventh inning and the Washington Nationals roughed up Oswalt in his Philadelphia debut, halting the Phillies' eight-game winning streak with an 8-1 victory Friday night.

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"I came out amped up, held the ball a little tight," said Oswalt, who absorbed his major league-leading 13th defeat. "Didn't really start feeling better until the last two innings. Didn't really go my way."

Oswalt (6-13), a former 20-game winner acquired from Houston in a four-player trade Thursday, allowed five runs -- four earned -- and seven hits in six innings. He walked two, struck out four and hit two batters.

Pitching in a meaningful game in the NL East, where the Phillies are chasing first-place Atlanta, instead of for the fifth-place Astros was a big change for Oswalt.

"It's different for sure," he said. "This team is good from top to bottom. It's good to get on a pitching staff with [Roy] Halladay and [Cole] Hamels and the other guys. It's hard to gain 30 games overnight."

It was another tough start against Washington for the right-hander, who lasted only 2 1/3 innings, yielded four runs and was ejected for arguing balls and strikes when he faced the Nationals in Houston on May 31.

Opinions on Oswalt's effort varied -- even in the Nationals clubhouse.

"I don't think Roy was as sharp as he normally is," Washington's Adam Dunn said.

Added Nyjer Morgan: "Same Oswalt. He's got electric stuff. We just jumped right on him."

Phillies manager Charlie Manuel isn't worried about how Oswalt transitions to his new environment.

"His stuff was good. [Oswalt] was having a little trouble with his command. ... They kind of ambushed us from the start," Manuel said. "I like his stuff. He's going to be fine."

Stammen (3-4) went 6 1/3 innings, and gave up one run and five hits. He walked one and struck out five.

"I just got in a groove and had a pretty good sinker," Stammen said. "We got a lead so it was pretty much throw that over the plate and see if they could hit it. For the most part, I did all right tonight."

Jayson Werth homered for the Phillies.

Adam Kennedy had a season-high four hits for the Nationals, who have won three of four.

Morgan led off the Washington first with a triple and scored on Kennedy's groundout for a 1-0 lead.

Phillies catcher Carlos Ruiz's throwing error set the stage for two runs in the Nationals third. Stammen singled, Morgan was hit by a pitch and Kennedy tried to bunt the runners over. Ruiz pounced on Kennedy's bunt single and threw to third, but third baseman Greg Dobbs had left the base uncovered to charge the bunt and Ruiz's throw sailed into left field.

"[Dobbs] is supposed to hang there at the lip of the grass and see what's going on," Manuel said. "[Ruiz] was throwing to the bag. The only problem was no one was standing there."

Stammen scored and Morgan went to third on the error. Ryan Zimmerman hit a sacrifice fly for a 3-0 lead.

The early lead put Stammen at ease.

"You don't have to be so perfect and worry about one mistake costing you a big lead and the momentum of the game. ... As long as we kept the momentum of being ahead and being the aggressor, that helps the team as a whole," he said.

Josh Willingam doubled in two runs in the fifth after Kennedy doubled and Oswalt hit Dunn with a pitch.

Werth led off the Phillies seventh with a solo shot to left, his 15th homer of the season and second since June 10.

In the Washington seventh, Roger Bernadina hit a two-run, bases-loaded double and Ian Desmond added a sacrifice fly.

Notes

• Philadelphia SS Jimmy Rollins returned to the lineup after missing three games with a bruised left foot.
• Nationals manager Jim Riggleman said he would employ a closer-by-committee approach following the trade of RHP Matt Capps to Minnesota on Thursday night.
• RHP Stephen Strasburg, who was scratched from Tuesday's start and placed on the 15-day DL on Thursday with right shoulder inflammation, will play catch in the outfield either Saturday or Sunday, Riggleman said.
• Oswalt had a forgettable first at-bat as a Phillie. Bernadina charged what might have been a single to right and threw Oswalt out at first.
• Philadelphia's Placido Polanco went 0 for 4, ending a 24-game hit streak against Washington. It was the longest active streak against an opponent in the majors.
• With 2 2/3 innings of spotless relief, the Washington bullpen now has 14 1/3 scoreless innings over four games.