WASHINGTON -- The numbers said it all for Manny Acta on Saturday night: 30 outs -- far too many, in the manager's eyes -- two errors and two unearned runs that turned out to be the difference in a 6-4 defeat at the hands of the Houston Astros.

"We had our chances; we just played poor defense," Acta said. "I don't think that, with the offense we have, we can afford to be giving 30 outs to the opposition."

Acta was referring in part to a Ronnie Belliard error, committed at the start of a three-run fifth inning, that ended up costing the Nationals a chance at their first back-to-back wins in two weeks.

The fifth inning began when Darin Erstad reached on the error, and it ended eight batters later. In between, the Astros used five hits to plate three more runs and drive starter Collin Balester from the game.

Belliard's error wasn't the only fielding gaffe to hurt the Nationals in the fifth. With one out and runners at the corners, first baseman Paul Lo Duca fielded a grounder near the bag, stepped on first and loaded up to throw toward third to try to catch Lance Berkman in a rundown there.

At the same time, Carlos Lee was running back to first, apparently confused by the situation, and Lo Duca could have tagged him for the third out. Lee made it back to the bag safely, however, and Berkman scored a batter later.

When asked, Acta said he wasn't sure what Lo Duca thought had happened on the play.

The teams traded first-inning runs, with the Nationals playing small ball to get on the board. Willie Harris led off the first with a hustle double to right. He moved to third on Lo Duca's groundout and scored on a sacrifice fly by Cristian Guzman to even the score at 1-1.

Washington kept the runs flowing an inning later when Belliard continued his recent tear with a double to left-center. He scored one batter later on a Jesus Flores single.

However, the Astros answered with two runs in the fourth and the three in the fifth, giving them a commanding 6-2 lead and ending Balester's night.

After the game, Balester, who lasted just 4 2/3 innings and surrendered six runs, four earned, said he struggled to "drive" his offspeed pitches down, which led to problems with his command and keeping his pitches down.

"I was flipping it up there," Balester said. "I've just got to get more consistent in throwing those same pitches instead of easing up on it."

The Nationals rallied for two runs in the sixth and threatened to score more. After Harris led off the inning with his fifth home run of the year, Washington loaded the bases with just one out and scored when reliever Chris Sampson hit Flores with the first pitch he threw.

However, Wily Mo Pena grounded into a 1-2-3 double play on the next at-bat, ending the rally. Houston's bullpen closed Washington down the rest of the way.

Acta admitted Pena's at-bat "took all the air out of" the Nationals, who put just one runner on the rest of the game. He characterized Pena's offensive struggles this season after the game, saying his left fielder looked "lost." Acta also said the training staff is looking at Pena's left shoulder, which has bothered him.

"Right now, [Pena's] just lost at the plate," Acta said. "He looks lost at the plate."

Hitting coach Lennie Harris said he thought Pena's struggles have a lot to do with his shoulder, which Harris said has been hurting him most of the year. He said the right-handed power hitter has trouble getting around on pitches inside, which Harris said were the kind of pitches Pena used to jump on for home runs last year.

"Wily Mo's hurting," Harris said. "I could be wrong, but to me, he hasn't been the same."

Harris said Pena is in the batting cage working every day, so much so that Harris said he has to "run him out." Harris called Pena "brave" for playing through his pain this year, adding that the slugger told Harris he might need a cortisone shot in the shoulder soon.

"It's hard for me to answer his questions, because I don't know his pain, what he's going through," Harris said. "But just by looking at it, I can just see that he's frustrated, because he's not doing the things that he wants to do. It's hard to play when you're hurt."