Lannan didn't allow a run in 5 2/3 innings to improve to 3-0 on the year.
In the second inning, New York had runners on first and second with no outs, but Lannan managed to get out of the inning. Jason Bay hit into a force play, Andres Torres flied out to Roger Bernadina in left, and Matt Harvey grounded out to end the threat.
"I was trying to go out there and feel for my two-seam [fastball], and I got a pretty good feel for it from the get-go. So I worked with that pretty much all game," Lannan said.
Johnson felt Lannan had a lot left in the tank to go further in the game. But the last time Lannan pitched was Aug. 30 for Triple-A Syracuse, so Johnson decided to rely on his bullpen.
The Nationals used six relievers, with Drew Storen picking up his second save of the season.
"How about John Lannan? That's was another great effort. He has three great games for us. He pitched a superb ballgame," Johnson said. "With that long delay in-between his starts, I didn't want to take a chance with him [going further]. The bullpen held up."
In some ways, it's another audition for Lannan, because he is on the bubble when it comes to making the postseason roster. The team needs only four starters in a series.
According to a baseball source familiar with the team's thinking, it seems unlikely that Lannan would make the postseason roster unless a pitcher like Sean Burnett couldn't play because of injuries. But Burnett returned to action Wednesday after missing time with elbow soreness.
The Nationals already have long relievers in Craig Stammen and Tom Gorzelanny, so it would seem Lannan is not a fit there either. The Nationals have not made an announcement on who will be on the postseason roster in October.
It has been an up and down season for Lannan. He spent most of the season in the Minor Leagues after he was beaten out of a rotation spot by left-hander Ross Detwiler at the end of Spring Training.
Prior to Wednesday's game, Lannan made two spot starts for the big league team -- both in doubleheaders -- and won each start.
On Wednesday night, it was a different story, as Lannan took over Strasburg's turn in the rotation.
"Tonight, [the Mets] were licking their chops. They are finally getting to face a starter that is not throwing 95, 96 [mph]," shortstop Ian Desmond said. "He comes down and shuts them down for 5 2/3. He definitely could have went longer. ... John looked great. He did his job. That's what we should expect from him the rest of the way.
Lannan acknowledged that it is sometimes hard to be true to himself on the mound. While the rest of the rotation throws 95 mph or better, Lannan is a finesse pitcher, who throws in the high 80s, low 90s.
"I'm not going to say I went out there throwing 96, but this rotation is different," he said. "There is something about it every five days. When Stephen was doing it, he was just different. I just go out there -- not try to do what they do -- and I do like to compete ... and I can do that."
Harvey started for New York and was dominant for most of his night. He allowed a run on five hits and struck out 10 batters over five-plus innings. The run against Harvey was in the fourth inning, when Ryan Zimmerman hit a 1-0 pitch over the left-center-field wall to extend his hitting streak to 16 games. Zimmerman also has driven in at least one run in nine consecutive games.
Zimmerman believes Harvey is going to be a productive power pitcher one day.
"I think that kid is going to be really good -- one of those big power pitchers -- once he figures out how to throw his other stuff for strikes a little bit more," Zimmerman said. "That's a good fastball. He mixes it up with a little cutter. It didn't look like he had a real good feel for his curveball, so he eliminated that pitch."
The Nationals added to their lead in the eighth inning, when Desmond hit his 23rd home run, a solo shot over the left-field wall, off reliever Jon Rauch.
The Nationals now go to Atlanta to play a three-game series against the second-place Braves.
"We have a big series coming up Friday. We still need to show them we are supposed to be where we're at," Johnson said.