Storen, who has been sidelined all season following right elbow surgery in April, was expected to be activated from the disabled list Friday, but those plans were put on hold following his rehab outing for Class A Potomac on Thursday.
Storen threw two innings Thursday and had good command, but his arm felt tired in his second inning of work, Nationals manager Davey Johnson said.
"The consensus from the coaches that watched him is that his arm strength isn't quite where it needs to be," Johnson said.
The plan now is for Storen to take Friday and Saturday to rest before making another two-inning rehab outing Sunday, this time for Double-A Harrisburg. The hope is that following that outing, the Nationals will be able to activate Storen.
Storen recorded 43 saves for Washington last season while posting a 2.75 ERA and striking out 74 while walking 20.
"I think Drew is champing at the bit, because he's been out all Spring Training and the first three months of the season," Johnson said. "Our concern is that he's ready and he doesn't have to [put] extra stress on his arm when he gets up here. We're just making sure his arm is ready before, so we don't have any problems once he gets here."