Bumgarner isn't fazed by hostile postseason environments. He's pitched so many huge playoff games already, and he's still only 25.
Joe Panik is a more than capable replacement for Scutaro. Gregor Blanco is not a great replacement for Pagan. He's had his own post-season magic moments, but his bat is a liability when he has to start every day. I'd say Peavy has replaced Cain very well so far.
Romo has not been so reliable lately, hence his demotion to 8th inning duty. Casilla is more than capable in that closer role. But there are other glaring holes (Left field? Starting pitchers not named Bumgarner or Peavy?).
The Nats are loaded with talent up and down their roster. The Giants just know how to get it done in October and have dispatched many "superior" teams. I'm not very optimistic this time around, unless one or two more of the potential starting pitchers can step it up.