Given how close the two teams' winning percentages are--.525 for the Sox and .445 for the Nats--I think taking 2 of 3 is a reasonable expectation for the good guys.
Right now the Sox have their best active roster in a long time--
Five decent starters (Paxton, Sale, Bello, Pivetta, and Crawford) plus a sixth, Houck, o/a 21 August, and at least seven decent relievers (Martin, Winck, Bernardino, Jansen, Murphy, Schreiber, and Whitlock). Bello might be tired, but no one else should be. In fact, quite the opposite.
Also six decent lefty bats (Devers, Yoshida, Casas, Duran, Dugo, and Wright) and six decent righty bats (Turner, Duvall, Story, Refsnyder, Reyes, and Wong).
So to me it's simply a matter of which Sox team shows up tonight at Nats Ballpark (which has good sight lines) at 7:05 tonight--the guys who can hit and pitch and even play defense, or the weak-hitting stumblebums we've seen too much of lately.