Horrible news for the Giants. Bumgarner sprained his shoulder and ribs in a dirt bike accident and is expected to miss 6-8 weeks.
Samardzija and Cain are going to really have to step it up for the Giants to even have a prayer. Cain was great for many years before faltering starting in the (I think) 2013 season, but his last 2 starts have seen him maybe going back to his old self finally. Samardzija ruled for the first half of last year, then sucked for the second half and most of this season so far. His last start was pretty good though. He struggled in the first inning but improved after that and had a good game except for the first inning. So we'll see.
I have no idea who'll take Bum's place in the rotation. If Chris Heston is still in the Giants organization, maybe they can call him up again. He had a no-hitter in his rookie season with the Giants (Can't quite remember what year that was... 2015 maybe?)
EDIT: Looks like rookie Ty Blach gets the first crack at Bumgarner's spot in the rotation.