Teams are now allowed to add an extra player for doubleheaders, and Lannan has gotten the call both times the Nationals have played twin-bills.
On Friday, he pitched into the seventh inning of the opener and allowed only one run in six innings of Washington's 7-4 victory over the Miami Marlins.
When the season began, Lannan was the odd man out and sent to Triple-A Syracuse. The Nationals' starting pitching has been sturdy and successful. None of the five pitchers who started the season in the rotation has been hurt, and Lannan has been toiling in the minors.
"I understand the situation. Going down there and pitching was a little weird," Lannan said.
Lannan is 6-10 with a 5.24 ERA at Syracuse. In these two spot starts, he is 2-0 with a 3.46 ERA.
He allowed three runs and four hits in six-plus innings. After walking five and striking out four, he was sent back to Syracuse.
"I wish I could stay up here, but I know the deal," Lannan said.
LaRoche had a first-inning RBI single, a third-inning home run, and a bases-loaded walk in the fourth. He has an 11-game hitting streak and is batting .463 over that stretch.
DeRosa, who missed nearly two months because of an oblique injury, doubled his RBI total for the season with a two-run single in the first-inning and a bases-loaded walk in the fourth.
The Nationals scored seven runs off Brad Hand (0-1), who had been recalled from Triple-A New Orleans to start the game. Hand, 1-9 in his career, gave up six hits and six walks in 3 2/3 innings. He was scheduled to return to the minors after the game.
"He was a little wild and got behind on the hitters," Miami manager Ozzie Guillen said. "If you are going to pitch in the big leagues you have to throw strikes."
Gonzalez allowed two hits and a run to start the ninth, and Tyler Clippard came in to earn his 22nd save in 24 chances.
Nationals manager Davey Johnson said he hoped Lannan would have gone a little deeper in the game.
"I thought he ran out of gas. I didn't get to him in time. Another big boost for us," Johnson said.
"He'll be back here soon," Johnson said of Lannan.
Johnson was most troubled by Storen's wildness. He walked two in just his eighth appearance since returning from April surgery on his right elbow.
"I didn't use some of the guys I had to use. Storen's ball was moving everywhere," Johnson said.
"I didn't want to do what I did there."
Miami scored a run in the first. Emilio Bonifacio led off with a walk, Donovan Solano singled, and Jose Reyes bunted for a hit to load the bases. Bonifacio scored when Carlos Lee hit into a double play.
LaRoche's RBI single and DeRosa's two-run single gave Washington a 3-1 lead in the bottom of the first.
"I was that close to being out of the first inning," Hand said.
LaRoche's 21st home run in the third made it 4-1, and the Nationals added three runs in the fourth on bases-loaded walks to LaRoche and DeRosa, and a single by Tyler Moore.
"It was a hard game, a hard night," Guillen said.
Reyes extended his hitting streak to 21 games. ... The Nationals acquired C Kurt Suzuki from Oakland for minor league C David Freitas. Johnson expects Suzuki to report in time for Saturday's game. To make room on the 40-man roster, Washington designated C Carlos Maldonado for assignment. ... The Marlins called up RHP Chris Hatcher to serve as the 26th man for the doubleheader. Hatcher pitched two scoreless innings. ... Miami 3B Donnie Murphy left the game in the seventh inning because of a strained left hamstring. He wasn't in the lineup for the second game, but is considered day-to-day.