Haren looks to win for the first time in more than five weeks, and Lannan comes off the disabled list to face his former team for the first time as Washington visits Philadelphia in the opener of a three-game series Monday night.
Lannan went 42-52 with a 4.01 ERA in 134 starts for Washington from 2007-12, but he pitched in the minors for most of last season and made only six starts in the big leagues.
The Nationals decided to move on and signed Haren to a one-year, $13 million contract, leaving Lannan to agree to a one-year deal with Philadelphia.
Haren (4-8, 5.70 ERA), though, is 0-5 with a 6.29 ERA in six starts since beating Detroit on May 9. He's especially struggled over his last two, allowing three homers and five runs in four innings of a 10-1 loss to the New York Mets on June 5 before giving up five more runs in five innings of an 8-3 defeat at Colorado on Tuesday.
"I feel like I am letting the team down. It's not for a lack of effort, it's not for a lack of work and not for a lack of belief,'' Haren said. "I believe every time I step on the mound I'm going to win and I have to keep that feeling. I've been good for a long time, this is just a bump in the road.''
The only start in which Haren didn't suffer the loss during his cold stretch was against Philadelphia on May 25, when he allowed three runs - including a homer to Domonic Brown - while striking out a season-high 10 in six innings of Washington's 5-3 defeat.
The right-hander is 0-3 with a 5.79 ERA in five career starts versus the Phillies - the only team he hasn't beaten.
Lannan (0-1, 6.14) has been on the DL with a strained quad tendon in his left knee since giving up six runs in 1 2-3 innings of an 11-2 loss to Cincinnati on April 17.
"With the injury, every day you look for something that got better from the day before," the left-hander told the team's official website. "I've been honest the whole process and I think I'm ready."
Manager Charlie Manuel believes he's ready, too.
"He's responding real well. He's throwing the ball good," Manuel said.
Lannan allowed four runs over 11 innings in two minor league rehab appearances and will face a Nationals lineup that was stymied in a 2-0 loss to Cleveland on Sunday.
Defending East champion Washington (34-34) had won five of its previous seven, plating at least five runs in those victories.
Cleanup hitter Adam LaRoche is mired in an 0-for-15 slump and is hitless with five strikeouts in 12 career at-bats against Lannan.
"Eventually, just like everything else in baseball, it will turn around," shortstop Ian Desmond told the team's official website. "We are not necessarily running out of time, but we need to start taking more steps in the right direction."
Howard, 7 for 16 in his last four games, is 1 for 9 with a homer and six strikeouts lifetime against Haren.
Washington has won six of the last eight meetings with Philadelphia.