Doolittle, who was named to the All-Star team, has 22 saves and a 1.45 ERA.
The move is retroactive to July 7, which means Doolittle will be ready to come back after the All-Star break.
Doolittle suffered the injury when he tripped on the mound recently and caught his toe, manager Davey Martinez said. He underwent an MRI over the weekend.
The Nationals also optioned right-handed reliever Jefry Rodriguez to Triple-A Syracuse and recalled right-handed pitchers Wander Suero and Austin Voth.