"I think every pitcher, if you ask them, they're not going to feel 100 percent every time," Strasburg said. "It's been affecting me more and more over the last three, four starts. It's a tough one to gauge because you want to go out and do your part. But today it just acted up on me a little more than the other ones."
Strasburg gave up a home run to Andrew McCutchen in the first inning. The 29-year-old has allowed 12 home runs this season, just one from tying his total in 28 starts last season.
The Giants began the second inning with three consecutive hits to add two runs. The Nationals' coaching staff then visited Strasburg on the mound, and after allowing another single, he escaped the frame. The Nationals caught a runner out at second base before Strasburg struck out pitcher Andrew Suarez and Joe Panik lined out.
Strasburg told Nationals manager Dave Martinez he wanted to pitch the third inning despite shoulder tightness, but Martinez didn't want to risk Strasburg's health. Strasburg, who threw 35 pitches, then left for the clubhouse with trainer Paul Lessard to end the shortest outing of his season.
Strasburg's fastball velocity Friday was about 95 mph, just slightly lower than his usual of 98, but he didn't look comfortable. Shawn Kelley replaced Strasburg but only threw one inning.
The Nationals placed Jeremy Hellickson on the disabled list Tuesday with a mild right hamstring strain, so if Strasburg's injury turns out to be serious, it would further complicate Washington's rotation.
The Nationals could use a four-man rotation through June 19 because of off-days. Erick Fedde, the Nationals' top pitching prospect, pitched for Triple-A Syracuse on Friday, meaning he could fill Strasburg's spot in the rotation, if needed. Fedde last started for the Nationals on May 23, when he gave up three runs over 5 2/3 innings against the Padres.
Strasburg has endured an array of injuries during his career in Washington, including forearm stiffness last season that sidelined him nearly a month. Some of his other injuries have included right elbow soreness, an upper back strain, an oblique strain and a lat strain. Strasburg went to the disabled list for right shoulder inflammation in 2010 shortly before the team discovered he needed Tommy John surgery for a torn UCL.
"I want to get the results of the MRI [Saturday] before we make any conclusions," Martinez said. "Hopefully it's nothing at all, just a little tightness, little inflammation, and we can take care of it."
Kyle Melnick is a reporter for MLB.com based in Washington.