The world number nine shot into the ATP Tour's top 10 after reaching the Wimbledon semi-finals in July.
The Bulgarian was set to be seeded third for the ATP 500 event, which is one stepping stone leading into the US Open, starting in late August.
Dimitrov was a quarter-finalist in Washington in 2013, when he lost to Tommy Haas, and Australian lucky loser Samuel Groth is set to replace him in the '14 main draw.
France's Julien Benneteau was also benefitted from Dimitrov's withdrawal, assuming the 17th seeding and receiving a bye past the first round.