By my count, there 17 people that if healthy, would be absolutely impossible to leave off. There is no combination of other players that could substitute for them.

Mortal Locks (17):

C - Wilson Ramos
C - Jose Lobaton (S)
1B - Adam LaRoche (L)
2B - Asdrubal Cabrera (S)
SS - Ian Desmond
3B - Anthony Rendon
3B/LF/1B - Ryan Zimmerman
OF - Bryce Harper (L)
OF - Denard Span (L)
OF - Jayson Werth

SP - Stephen Strasburg
SP - Jordan Zimmermann
SP - Doug Fister

RP - Tyler Clippard
RP - Drew Storen
RP - Aaron Barrett
RP - Matt Thornton (L)

...And The Rest:

Position Players (Pick 2 or 3)

C - Sandy Leon
1B/2B/3B/LF - Kevin Frandsen
2B/SS - Danny Espinosa (S)
1B/LF/RF - Tyler Moore
LF - Scott Hairston
LF/RF - Nate Schierholtz (L)
OF - Michael Taylor
OF - Stephen Souza
OF - Jeff Kobernus

Pitchers (Pick 5 or 6)

SP - Tanner Roark (Can be moved to bullpen for playoffs)
SP - Gio Gonzalez (L) (Can be moved to bullpen for playoffs but probably won't be happy about it)

RP - Rafael Soriano
RP - Ross Detwiler (L)
RP - Xavier Cedeno (L)
RP - Craig Stammen
RP - Jerry Blevins (L)
RP - Blake Treinen
RP - Ryan Mattheus