WASHINGTON - Nationals left fielder Jayson Werth is expected to play seven innings at Class-A Potomac on Thursday to start a minor league rehab assignment.

Werth began the season on the 15-day disabled list after having surgery on his right shoulder in January. He's eligible to come off the DL on Saturday, but manager Matt Williams said Wednesday "that date would probably be stretching it."

Williams said the team wants Werth to play nine innings in a game and on back-to-back days before joining the Nationals.

Another everyday outfielder on the DL, Denard Span, is going to take "a little bit longer" to be ready to play for Washington, according to Williams. Span, who had core muscle surgery in the off-season, will head to Florida this week to play in simulated games.

Both Werth and Span took simulated at-bats at Nationals Park against pitchers from Potomac on Tuesday and Wednesday.

The return of right-handed reliever Casey Janssen from the DL was slowed as he deals with tendinitis in his right rotator cuff. Janssen was slotted to be Washington's eighth-inning setup man.

"We thought about starting a throwing program, and it was a little tender, still so we backed off that a little bit," Williams said.