Brewers manager Craig Counsell said Peralta will undergo an MRI on Monday and will be heading to the injured list.
"Hopefully, it's nothing that big or crazy, but it's part of the game," Peralta said. "We'll see what happens."
Peralta (3-2) left after allowing a run-scoring double to Lane Thomas during the Nationals' six-run outburst in the fourth inning.
"I thought in the second inning, he didn't have the same life on his fastball," Brewers catcher Omar Narvaez said. "In the third inning, I went out to see how he felt. ... I asked him how he was feeling and he said he was a little tight. I told him there's no point this early in the season trying to be the hero."
Washington's six runs in the fourth marked its most productive inning of the season and matched the biggest run total the Brewers had allowed in a single inning this year. Peralta, a 2021 All-Star, entered the day having allowed just one run over 12 2/3 innings in his previous two starts.
The Nationals entered the day having dropped seven of their past nine, and they had scored a total of six runs in the seven losses. But they had an early 8-0 lead Sunday and cruised to the win in their final game before starting a seven-game homestand.
"Maybe that'll spark something and we can get some wins over the next few weeks here and get back in it," Thomas said.
Milwaukee didn't retire anyone in the fourth until Nelson Cruz -- the eighth batter of the inning -- got thrown out trying to stretch a bloop single down the right-field line into a double. The Nationals' first eight batters of the inning all got hits, tying a Washington franchise record for consecutive plate appearances with a hit.
"I think when guys are on a roll, they're throwing strikes," Thomas said. "I think we were just aggressive on pitches in the middle of the plate. That was my game plan. Pretty good fastball. It plays up a little bit. I was just going to be aggressive on the fastball and drive some runs in."
The Brewers begin an 11-game, 10-day trip Monday in San Diego. Adrian Houser (3-4) will start for Milwaukee.
The Associated Press contributed to this report.