Lannan was 2-5 with a 5.76 ERA in 14 starts. He didn't get out of the fifth inning in any of his last three starts, giving up at least 10 hits and five runs in each outing.
"I think he was starting to press," Nationals general manager Mike Rizzo said. "I think his struggles were getting the best of him. Psychologically, I think he was very worried he was letting his team down, that he wasn't giving his teammates a chance to win games."
Lannan also started the opener for the Nationals last year and led the team that season with nine wins.
The Nationals purchased the contract of right-handed pitcher Joel Peralta from Triple-A Syracuse on Monday.
Peralta was 2-0 with a 1.08 ERA and 20 saves with Syracuse. He's 4-11 with a 4.61 ERA in 221 major league appearances with the Los Angeles Angels, Kansas City and Colorado.
The Nationals did not say how long Lannan was expected to spend with Harrisburg. Washington manager Jim Riggleman said the most important factor would be how many ground balls the sinkerball pitcher was getting, rather than statistics.
"The best thing for John is to send him out and see if he can get straightened out down below," Riggleman said. "It's tough to try to work out mechanical things in the big leagues."
Rizzo did indicate he expected Lannan to return to the Nationals.
"He's a major league pitcher. He's going to be part of this rotation for a long time," Rizzo said. "The sooner he irons things out and gets comfortable, the better."