Lannan says he started feeling discomfort three starts ago in a game against the Colorado Rockies, mainly when throwing his sinker and changeup. A MRI exam revealed swelling around his flexor tendon.
Lannan said the problem got worse during his last start on Sunday, when he lasted five innings and allowed six runs in a loss to the Florida Marlins.
"The last start, I knew something was wrong," Lannan said. "I didn't have the life I usually have to get groundballs."
Lannan, the Nationals' Opening Day starter, is 1-2 with a 6.34 ERA in six starts. He has lasted past the sixth inning only once.
Lannan will rest for five days before starting to throw again.
"For whatever reason, he's just not sharp," manager Jim Riggleman said. "We know that he's not physically able to throw the pitch with total confidence."