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Edwin Jackson has the hardly damaging distinction of having the worst ERA in the Washington Nationals' rotation, but he's still been fairly solid in his first six starts with his new team.

During the Cincinnati Reds' first look at him, he was much more than that.

Less than a month after two-hitting the Reds in the nation's capital, Jackson looks to lead the Nationals to a three-game sweep of Cincinnati on Sunday afternoon -- though he'll have to get used to a new batterymate.

Washington's rotation has a 2.24 ERA -- far and away the best in the majors -- and four of those starters own top-15 ERAs in the NL.

The only one missing from that group is Jackson (1-1, 3.49 ERA), who's delivered a quality start every other time he's taken the ball. His latest was Tuesday in Pittsburgh, where he gave up two solo homers and only one other hit over seven innings before the Pirates won 5-4 on a walk-off homer.

His best start so far came April 14 against Cincinnati. Jackson gave up only two hits in his fifth career complete game, needing just 92 pitches in the 4-1 victory.

"We've never seen him that fine," Reds manager Dusty Baker told MLB.com after that game. "Usually he has a wild period in there some place, but he didn't have that at all today. I even asked the umpire one time, 'Are we that bad or is he that good?' He said, 'Hey, he was that good.'"

While the Nationals (21-12) have to feel comfortable no matter who is on the hill, they have a major potential concern looming behind the plate. Catcher Wilson Ramos left Saturday's 2-1 win over the Reds (16-16) in the seventh inning after his right knee buckled while he chased a passed ball.

Results of a postgame MRI weren't immediately made available, but manager Davey Johnson wasn't optimistic.

"I have a bad feeling he tore something and it's going to be a while," Johnson said. "He's really down and still in a lot of pain. He can't put any weight on it. Not good."

That's the latest blow for an offense that's already dealing with long-term injuries to Ryan Zimmerman (shoulder) and Michael Morse (lat). Though the Nationals lead the NL East by one-half game over Atlanta, they're hitting just .201 over their last nine road games.

Bryce Harper's average, meanwhile, is down to .213 as he's gone 0 for 9 in the series with four strikeouts and 10 stitches from his post-at-bat dugout tirade Friday.

Washington may not find hits any easier to come by against Bronson Arroyo (2-1, 2.75), who was nearly as good as Jackson when the teams got together last month. The right-hander held the Nationals to three hits over 7 1/3 scoreless innings in a 2-1 loss in 13 innings on April 13.

His experiences against Washington have tended to be all-or-nothing outings. In 10 career starts, Arroyo has pitched at least six scoreless innings five times and given up two runs over eight in another. In the other four, he's 0-3 with a 13.50 ERA.

He didn't struggle at all Monday in Milwaukee, holding the Brewers to one run over 6 2/3 innings while striking out a season-high nine in a 6-1 win.

That was a rare good night for the Reds' offense. After averaging 4.54 runs last season -- seventh in the majors -- Cincinnati has put up just 3.75 per game in 2012 to rank among baseball's bottom third.

It's especially struggled against Washington. The Reds have totaled eight runs in their five losses to the Nationals, and they'll need to win Sunday to avoid finishing the season series 1-6.