The Nationals are planning to place outfielder Jayson Werth on the 15-day disabled list following tonight's game against the Cubs 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 Rizzo expressed confidence Werth would be completely healed by then.

The Nationals have 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 Nationals plan to wait until after tonight's series opener against the Cubs to officially place Werth on the DL and then will promote a replacement outfielder from Class AAA Syracuse: either Eury Perez or Jeff Kobernus.

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. Kobernus, selected by the Tigers in the Rule 5 draft but then returned to the Nationals at the end of spring training, 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.

Bryce Harper, meanwhile, also is out of the Nationals' lineup tonight after having a procedure to remove an ingrown toenail.