This move might not hurt the Giants as much as you might think. For all Sandoval's popularity and postseason heroics, he's really had just two of seven MLB seasons anywhere near All-Star quality (2009 and 2011, oddly, not years the Giants won the World Series). The rest of his career is just a bit above average (best of the other five: 3.3 bWAR in 2014, compared to 4.3 bWAR in 2009 and 6.1 in 2011).
According to their depth chart, the number one candidate to replace him is ex-Cub Casey McGehee. This would be a significant downgrade, although McGehee had a decent 2014 after a year playing in Japan. Giants prospect Matt Duffy, who is mostly a second baseman and shortstop, might get some time there, too, in spring training.
Otherwise last year's World Series champs return intact, save the addition of former Royals outfielder Nori Aoki, who will play right field while Hunter Pence is out with that broken arm he suffered last week. After Pence returns Aoki will likely back up all three outfield positions.
The only questions in the Giants' returning rotation is whether Tim Hudson can have one last good year at age 40, and if Ryan Vogelsong can have a better year than he did in 2014. Madison Bumgarner, so great in last year's postseason, should be entering his prime at age 25, and so should Buster Posey, who will turn 28 in a couple of weeks.
Basically, the Giants are a strong contending team in a division that could have three such teams. But since this is an odd-numbered year, it's not their year to win the World Series again. And yes, I know that's an illogical conclusion, but it is very odd that they have won three World Series in the last five years, all in even-numbered seasons.
The Cubs won't face the Giants this year until August. The Giants will visit Wrigley Field for a four-game series August 6-7-8-9 and the Cubs travel to San Francisco three weeks later, August 25-26-27.