The Nationals have also dropped Dan Haren's last 10 starts heading into Monday night's opener of a four-game home set with the Pirates.
Pittsburgh (57-39) totaled 10 runs and went 2 for 27 with runners in scoring position while losing two of three over the weekend in Cincinnati. The Pirates are hitting an NL-worst .225 with men in scoring in position.
Washington (48-50) also struggled in that department this weekend, going 2 for 26 and scoring five runs while getting swept at home by the Dodgers. The Nationals are batting .237 with runners in scoring position.
"I got confidence in 'em," manager Davey Johnson said. "I don't want to keep talking about the offense. That's all we seem to talk about, that and the injuries."
The Nationals will be out to stop their three-game slide as well as this losing streak with Haren (4-10, 5.61 ERA) on the mound. Haren is 0-7 with a 5.93 ERA during the longest winless stretch of his career.
The right-hander, however, has yielded two runs over 11 innings in two starts this month. He feels a stint on the disabled list to rest an inflamed shoulder has put him back on the right track.
"I kept the ball down," he told the Nationals' official website. "That has been the key for me in the last two starts."
Haren hasn't faced the Pirates since 2010. Catcher Russell Martin has the most experience against him, going 7 for 39 (.179).
Pittsburgh has a slight edge in homers over Washington this year, 95-88. The Pirates hit six solo homers over the weekend while the Nationals ended a six-game drought when Jayson Werth hit a pair in Sunday's 9-2 defeat.
Garrett Jones homered for the second straight day for Pittsburgh in Sunday's 3-2 win over the Reds. Jones had not gone deep in 10 games before these last two.
"I'm just one of the pieces that need to pick it up,'' Jones said. "I had a slow first half and this is the time to hit and be productive and help your team win."
The Pirates avoided a sweep and extended their lead over the third-place Reds to three games.
"There's still a lot of games left, but to get this final game is big," Jones said. "Every game matters."
Pittsburgh will start Charlie Morton (1-2, 3.19), who is 0-2 with a 4.11 ERA in three outings versus Washington but hasn't faced the club in two years.
The right-hander, making his seventh start after elbow surgery, gave up two runs over a season-high seven innings July 12 in a 3-2, 11-inning win over the Mets.
"It's good to see him continue to get a little bit better every time he has the ball," manager Clint Hurdle told the Pirates' official website.
Morton has never faced Bryce Harper, who was 0 for 4 with three strikeouts Sunday and is hitting .206 in 17 games since returning after missing a month with left knee bursitis. Harper has struck out 11 times in his last 23 at-bats.
The Nationals took two of three in Pittsburgh from May 3-5.