According to Roster Resource, the Giants' current starting rotation is Madison Bumgarner, Jake Peavy, Matt Cain, Chris Heston, and Clayton Blackburn. Ty Blach and Chris Stratton would probably be the first lines of defense in case of injury or ineffectiveness. There's no Mike Leake, no Tim Lincecum. No Tim Hudson, and no Ryan Vogelsong. If the Giants don't sign another starting pitcher or two, the already-thin rotation will become unconscionably thin.
They will sign another starting pitcher or two.
There's an outside chance at some sort of wacky trade, but the simplest solution is the most likely solution.