Lannan had started 14 games this season, but accumulated just a 2-5 record with a 5.76 ERA while giving up 17 earned runs in his last four appearances - most recently a loss to the Chicago White Sox Sunday.
Lannan said after Sunday's game that his arm was fine, but his pitches just weren't hitting their spots.
"I'm letting my team down. I'm letting myself down. I'm letting everybody down," Lannan said. "The ground balls (I got), I was still missing my spots. Just because they were ground balls doesn't mean I was hitting my spots. They were hits. It's just not a run of bad luck. It's just, I'm just not making my pitches. From the get-go, I threw a lot of strikes, but I wasn't necessarily hitting my spots. So I've just got to go back and work harder."
The Nationals have called 34-year-old Joel Peralta up from Triple-A Syracuse to take his place.
Peralta has accumulated a 1.08 ERA, a 0.93 WHIP and a 38/7 K/BB ratio in 28 appearances for Syracuse this season and is expected to work in middle relief.
Goessling also reports that Ross Detwiler has been removed from the 60-day disabled list and will be assigned to Harrisburg and that Garrett Mock has been moved from the 15-day to 60-day disabled list due to the cervical spine injury that's hampered him since early April.