The Washington Nationals’ lineup confronted a journeyman starting pitcher Saturday evening, and it still demanded near perfection out of Jordan Zimmermann. His offense allowed him no margin for mistakes, and in a crucial moment he made a decisive error on a simple play. Zimmermann remained unbeatable throwing the ball over the plate, until he turned and threw it to first. After a 33-year-old lefty named Eric Stults shut down the Nationals for eight innings, their 2-1 loss to the San Diego Padres hinged on Zimmermann’s second mistake of the day. Trying to stifle a rally before it started, Zimmermann rifled a pickoff attempt into foul ground. The next batter, Everth Cabrera, cracked a go-ahead single into center field. It held as the game-winner after the ninth inning brought another fruitless frame for the Nationals’ sputtering hitters. It lasted 2 hours 1 minute, and everybody went home in time for dinner. Zimmermann allowed two runs, one earned, on seven hits and no walks in an 85-pitch, eight-inning complete game, lowering his ERA to 1.62. He still could not become the first major leaguer to reach eight wins. Zimmermann also singled, one of the meager four hits, all singles, the Nationals managed. “He pitched an unbelievable game and swung the bat better than most of the guys in the lineup,” Manager Davey Johnson said. “It’s a waste we couldn’t pull this one out. That’s a tough loss.” The Nationals had scored six runs in each of the past two games, powered by a surging middle of the order. Their offense, dreadful all season, threatened respectability. But with Bryce Harper sidelined with a swollen left knee and their sluggers silenced, the Nationals had almost no other offense to speak of. They have a second baseman, Danny Espinosa, with a .197 on-base percentage, lowest in the National League. Tyler Moore, an outfield replacement with Jayson Werth on the disabled list and Harper shelved, finished the day hitting .136. Over the past two days, Nationals batters other than Harper, Ryan Zimmerman and Adam LaRoche are 5 for 42. “It’s tough to be consistent when you have guys in and out of the lineup all the time, myself included,” Zimmerman said. Nothing about Stults’s repertoire suggested he could oppress a lineup. He had a 4.57 ERA and had allowed 10.1 hits per nine innings this season, better than only 13 National League starters. He had stuck out 6.2 hitters per nine innings, well below league average. He throws his off-speed pitches — a slider, curve and change — more than half the time, and his fastball travels at 86 mph, not quite fast enough to send Doc Brown’s DeLorean back in time. In 2010, Stults started 21 games and posted a 5.07 ERA for the Hiroshima Carp.