Strasburg gave up four runs -- three earned -- on season highs of six hits and three walks over 4 2/3 innings and suffered his first professional loss Sunday as the Harrisburg Senators were beaten by the Altoona Curve, 6-1.
MLB.com reported on Saturday that the Nationals were expected to move Strasburg up to the International League, provided he had a good outing Sunday against the Pirates' Double-A affiliate.
But Strasburg appeared to struggle with the strike zone of plate umpire Jon Byrne before another sellout crowd of 7,619 at Metro Bank Park.
"Every umpire has a different zone. You've just got to keep pitching where you want to put it," Strasburg said. "I was doing that, I just wasn't getting the call."
After coasting through the first inning, he issued a leadoff walk in the second to Matt Hague and gave up consecutive singles to Jim Negrych, Miles Durham and Alex Presley.
None of the balls was hit particularly hard, with Negrych reaching on an infield single and Durham blooping a fly ball in front of center fielder Bradley Coon to score the game's first two runs. After walking Kris Watts, Strasburg fanned Justin Wilson, but a throwing error by second baseman Michael Martinez allowed two more runs to score.
"I did what I could do," the San Diego State product said. "I think they squared one ball up on me. It's just one of those days where you make a good pitch and you get weak contact [and] it just finds a hole."
Strasburg was in trouble again in the third after issuing another leadoff walk to Jordy Mercer -- matching his walk total for his first four starts -- and giving up a base hit to Matt Hague. But Negrych flied out and Durham bounced into an inning-ending double play.
The No. 1 overall pick in last year's Draft struck out two in a perfect fourth, then hit Chase d'Arnaud to open the fifth and gave up a bunt single to Gorkys Hernandez and another base hit to Mercer. After Hague grounded into a double play, Strasburg was done. He left to a standing ovation.
"It's been a great experience; it is a great experience here," he said.
The 21-year-old right-hander with a fastball that registers triple digits threw 79 pitches, just below his limit of 85. He struck out four but had his ERA rise from 0.52 to 1.64.
"There's always going to be days when things just aren't going your way," Strasburg said.
If he is promoted, he likely will face the Gwinnett Braves on Friday in Syracuse.
"All I know right now is that I'm going to show up to the field tomorrow, play some catch, get ready for my bullpen [session] and get ready for my next start," he said.