He started feeling discomfort in his elbow against the Rockies on April 21, when pitched six innings and gave up four runs on 11 hits. He picked up a no-decision in a 6-4 victory.
After the game, Lannan told the trainers about his problem and had an MRI. It revealed swelling in his flexor tendon, but no structural damage. Lannan was told he could still pitch, but he didn't have quality starts in his next two outings, allowing nine runs in 11 innings.
Lannan said he felt discomfort in the elbow while throwing his two-seam fastball and changeup, the two pitches he relies on the most. He said he knew something was wrong with the elbow in his last start against the Marlins.
"In my last start, I knew something was wrong," Lannan said. "I didn't have normal stuff. I didn't have the life that I usually have to get ground balls. I said something, so now I take the next step.
"Of course, it's frustrating. I've never been injured before. Hopefully, it gets better and this program works out. I'll skip my next start and be back for the next one."
The Nationals have not announced who will replace Lannan in his next start. Reliever Miguel Batista could get the start for Washington, which also could call up another arm for the bullpen. Reliever Drew Storen is not in the mix. Right-hander Joel Peralta and left-hander Doug Slaten, both from Triple-A Syracuse, are candidates.
Then again, the Nationals could call up a starter. Right-hander J.D. Martin of Syracuse and left-hander Matt Chico, who is pitching for Double-A Harrisburg, are candidates. Both have experience in the Major Leagues.