Updated at 11:00 p.m.
The Nationals are planning to place outfielder Jayson Werth on the 15-day disabled list Saturday morning with what general manager Mike Rizzo termed a "very slight strain" of his right hamstring.
Werth hasn't played since May 2 in Atlanta, when he felt the hamstring tighten up while tracking a ball in right field, so the DL move can be backdated. He'd be eligible to return on May 18, and he expressed confidence he'll be completely healed by then.
"We feel like we're close, and I feel like it's really not that severe," Werth said after the Nationals' 7-3 win over the Cubs. "But it's one of those things, you go out there with the way it is and you might make it severe pretty quickly. Erring on the side of caution, but at the same time ... I think it's necessary. Fortunately it'll only be another seven days."
The Nationals had been playing a wait-and-see game with Werth over the last week, hoping each day he'd be ready to return from what originally was termed nothing more than hamstring cramps. The club believed it might actually have been a result of dehydration from a stomach ailment, a condition for which Werth has since begun taking antibiotics.
But while trying to do leg curls this afternoon at Nationals Park, Werth's hamstring "went into spasm," according to manager Davey Johnson. Team doctors decided he would be best served taking the next week off to let the hamstring completely heal.
Rizzo said an MRI performed on Werth on Wednesday showed a "very slight strain," described by the GM as "a Grade 1-ish type of strain."
"The most frustrating thing about the whole thing is I feel pretty good," Werth said. "I was out there running the last couple days. I'm running good. It's just a little cramp there at times, and cramps lead to bigger and worse injuries."
The Nationals won't officially announce any roster moves until Saturday morning, but outfielder Eury Perez was scratched from Class AAA Syracuse's lineup Friday night and was on his way to Washington.
Perez, who saw minimal action with the Nationals last September, is hitting .346 with seven stolen bases in 23 games, though the speedy leadoff man has drawn only one walk so far this season.
Johnson earlier said the club was also considering Jeff Kobernus as a potential replacement for Werth. Selected by the Tigers in the Rule 5 draft but then returned to the Nationals at the end of spring training, Kobernus is a natural second baseman who actually has seen more time in the outfield this season as the organization tries him out as a utilityman. The 24-year-old is hitting .371 with 14 stolen bases.