Washington manager Davey Johnson said Strasburg experienced tightness in his pitching arm while throwing in the bullpen Thursday in New York before Washington's game against the Mets.
"Well, yesterday after he threw in the pen, he was experimenting with another pitch, and I don't know if that caused his arm to get a little tight, stiffness in his right forearm," Johnson said.
"It's not down where they did the Tommy John (surgery in 2010). It's on the soft tissue on the top side of the forearm."
The Nationals opened a 10-game homestand with Ross Ohlendorf (3-0) starting in Strasburg's place. He pitched the team to a 6-1 victory over the Phillies.
"As I was going outside to watch him throw he was coming back in," pitching coach Steve McCatty said. "He said his arm kind of, it was, you know, he felt like he couldn't throw, couldn't get loose. So we waited until afterwards and tried it again, and it just didn't feel good. So nobody wants to push it."
Johnson was asked if the team believed it was a muscular issue as opposed to involving a tendon or ligament.
"Yeah, it's muscular, soft tissue," he said.
Strasburg was examined by team medical director Dr. Wiemi Douoguih, and Johnson and McCatty said the plan is for Strasburg to rest a few days and take anti-inflammatories.
Johnson said Strasburg's next start is slated for Sept. 19 against the Marlins.
"I feel pretty confident," Johnson said of the likelihood that Strasburg will make that start.
Johnson declined to say which pitch Strasburg was experimenting with and the pitcher was not in the clubhouse while it was open to the media prior to Friday's game.
"I was standing there, OK? All pitchers play with grips all the time," McCatty said. "When they 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. Not trying to turn it over. But just throwing a certain pitch. And he didn't throw that many."
McCatty was uncertain if that's what caused the tightness.
"I don't know if that's it or not," he said. "You know, guys throw, they get irritation. It's unfortunate that it's him because it now becomes, `Oh my God, we landed on the moon.' So that's what it was. He's got some irritation."
Strasburg (7-9) went six innings in his last start on Sept. 8, allowing four runs and four hits in a win at Miami. He threw 94 pitches.
The Nationals have won seven straight and trail the Reds by 4½ games for the final NL wild card.