MILWAUKEE -- Since stepping into a Major League clubhouse for the first time a little more than four years ago, rarely has Freddy Peralta not had a smile across his face.

That smile was absent Sunday when the young right-hander turned from his locker to talk to reporters following an 8-2 loss to the Nationals at American Family Field in which Peralta was pulled in the fourth inning because of shoulder tightness that is expected to send him to the injured list.

"We'll see what happens tomorrow," said Peralta, who will remain in Milwaukee and undergo an MRI exam Monday while the Brewers open a three-game series at San Diego. "Hopefully, it's nothing that big or crazy but it's part of the game."

• Peralta exits with shoulder tightness, will get MRI

After a somewhat bumpy start to his season, Peralta came into the game on a roll. Over his past five starts, he was 3-0 with a 1.57 ERA and spun a gem his last time out when he struck out 10 batters -- including the 500th of his young career -- in seven scoreless innings against the Braves.

With the Brewers trying to close out their fourth series sweep of the season, Peralta looked like he was picking up where he left off against the Braves. He struck out his first two batters then worked around an error by Mike Brosseau to get out of the first.

The Nationals took a 1-0 lead against Peralta in the second when Maikel Franko reached on a two-out single and scored on Lane Thomas' double then added another run in the third when César Hernández, who drew a leadoff walk, scored on a two-out single by Nelson Cruz.

Things fell apart in the fourth as Peralta allowed three straight hits to open the inning. When pitching coach Chris Hook came out to check on him, Peralta let Hook know about the tightness in his shoulder, which led to a visit from manager Craig Counsell and the members of Milwaukee's training staff, ultimately bringing his day to an early end.

"He didn't have the same life on his fastball, so [we] went out to see how he felt," catcher Omar Narváez said. "He was trying to push, and the ball wasn't coming out like it normally does. 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. That's basically what happened."

The game slipped away quickly after Peralta came out. Washington went on to score five more runs in the inning after recording five straight hits against left-hander Brent Suter, while the Brewers' offense couldn't do much against Nationals starter Aaron Sanchez, who allowed seven hits and a pair of walks over five innings but just two runs in large part because Milwaukee hit into three double plays over the first four innings.

"We didn't play a good baseball game," manager Craig Counsell said. "It's only one game, so you have to turn the page, but losing Freddy hurts."

While an official roster move is forthcoming, the Brewers will stick with a five-man rotation in Peralta's absence for now, meaning left-hander Aaron Ashby, who's served as a bit of a swingman by working out of the bullpen and as a sixth starter over the first few weeks of the season, will slide into the spot vacated by Peralta.

"Ashby is gonna pitch on Wednesday in San Diego," Counsell said. "We're going to have to go with five guys ... so for now it doesn't change much, but we lose a good pitcher for at least a couple of weeks."

Ashby has made three starts this season, posting a 4.61 ERA with 13 strikeouts and 13 walks over 13 2/3 innings of work.

"He's been pitching really good, and I know that he's going to do what we're going to need now that I'm going to be out for a little bit," Peralta said. "He's been looking good, and I know that he's going to cover some innings."