To make room on their roster, the Nationals optioned outfielder Matt den Dekker to Triple-A Syracuse before Monday's game at Fenway Park.
Werth missed the first six games of the season after surgery on his right shoulder in January. He did a rehab stint at Single-A Potomac last week.
Den Dekker is 0 for 2 as a reserve for the Nationals this season. He was acquired March 30 in a trade that sent lefty reliever Jerry Blevins to the New York Mets.