Johnson said the tightness is in the soft tissue of Strasburg's arm, not the surgically-repaired ligament in his elbow.
"All of the doctors think it's a non-issue," Johnson said. "We'll give him some rest. There might have been some inflammation in there or something from some new muscles being used on this new pitch. Who knows?"
Strasburg was examined by doctors in New York on Thursday and team doctor Wiemi Douoguih on Friday. Johnson said Strasburg had some tests, but the skipper did not think that the right-hander had an MRI.
Strasburg was not available for comment before Friday's game, and Johnson would not reveal the new pitch with which the right-hander was experimenting. Pitching coach Steve McCatty said that Strasburg's experimentation was nothing out of the ordinary.
"All pitchers play with grips all the time," McCatty said. "When you play catch, you turn the ball different ways, you put pressure on a finger, whatever you do, you're messing around. That's all he was doing."
McCatty said that Strasburg only threw seven or eight of the new pitches. The pitching coach, who spent nine seasons in the Major Leagues, also pointed out that there's no way to know whether the tinkering caused the tightness in Strasburg's forearm.
"Guys throw, they get irritation," McCatty said. "It's unfortunate that it's him because now it becomes, 'Oh my God, we landed on the moon.' So, that's what it was. He's got some irritation."
Johnson, however, admitted that the news was initially worrisome.
"Yeah, I was concerned," Johnson said. "Any time a pitcher tells you he can't throw tomorrow, doesn't want to play catch [it's concerning]. He came in here and got treatment, the doctor in New York examined him and didn't think it was anything serious. [It is] probably just tight. So, here we are. He's not pitching. He'll pitch [Sept.] 19th."
Right-hander Ross Ohlendorf, who has made sporadic starts for the Nationals this season, started in Strasburg's place. Johnson said that Ohlendorf had been preparing as if he would start Thursday, so he effectively received an extra day of rest.
Strasburg has made 28 starts this season, the same amount he made in 2012 before the Nationals elected to shut him down for precautionary reasons. Last year, Strasburg went 15-6 with a 3.16 ERA in 159 1/3 innings. Though he is just 7-9 this season, he sports a 2.96 ERA in 170 innings.
While Strasburg's shutdown last year was carefully planned months in advance, his forearm tightness and missed start Friday were completely unexpected.
"Right out of the blue," Johnson said. "But Stephen's a perfectionist. He likes everything just perfect. So any time you've had an arm injury, any little discomfort and the alarm bells are going to go off."