Doolittle, out since Aug. 18 with right knee tendinitis, threw a simulated game Wednesday with mixed results, saying he felt fine physically but that his execution mechanically wasn't where he wanted it to be.
"I need to be sharper if I'm going to help this team when I come back," said Doolittle.
The Nationals did get bullpen help Friday, reinstating left-hander Roenis Elias (right hamstring strain) from the 10-day injured list and right-hander pitcher Hunter Strickland from the paternity list.
Washington optioned catcher Spencer Kieboom to Double-A Harrisburg and designated left-handed reliever Matt Grace for assignment.
Grace, the Nationals' eighth-round pick in the 2010 draft, went 5-4 with two saves and a 4.29 ERA in 178 career games for Washington. This season, he was 1-2 with a 6.36 ERA and one save in 51 games.