There was no obvious sign of trauma, but Peralta said he began feeling tightness in his leg during the third inning of what would become a 4-0 Brewers win at Nationals Park. After Bryce Harper bunted for an out leading off the sixth, catcher Martin Maldonado noticed Peralta adjusting his landing to compensate for the discomfort, and called for manager Ron Roenicke and assistant athletic trainer Dave Yeager to the mound.
Peralta insisted he was fine and remained in the game to bounce two more pitches, after which Roenicke returned to make a pitching change.
"He said he was fine, but when we saw him landing way on the first-base side," Maldonado said, "you don't want to risk him in a situation like that."
John Axford took over with Ryan Zimmerman in a 2-2 count and the teams locked in a scoreless tie.
Peralta allowed no runs on three hits in 5 1/3 innings, with two walks and five strikeouts. It was an excellent bounce-back after he surrendered seven runs (three earned) on eight hits in five innings against the Cubs last week.
"We had a long game in Pittsburgh [on Sunday] and a long game yesterday [for the bullpen], so you don't want to go out," Peralta said. "I didn't want to come out, but I have to."
Peralta is the third Brewers pitcher to be bitten by a hamstring issue this season. Starter Marco Estrada is currently on the disabled list with a left hamstring strain, and closer Jim Henderson spent two weeks on the DL in late May and early June with a right hamstring strain.
"When it's a pull, you wonder -- which Henderson and Estrada were. We're hoping this isn't a pull," Brewers manager Ron Roenicke said. "It doesn't sound like it is, so we're hoping in a couple of days he'll be all right. It was a great game for him. Great command of the fastball, great movement down, getting ahead of batters."
If he stays on schedule, Peralta's next start would be Sunday at Miller Park against the Mets, against whom he pitched the best game of his budding career last Sept. 16. Peralta surrendered only two hits in eight shutout innings that day.
Adam McCalvy is a reporter for MLB.com. Read his blog, Brew Beat, and follow him on Twitter at @AdamMcCalvy. This story was not subject to the approval of Major League Baseball or its clubs.