According to San Francisco Chronicle reporter Rusty Simmons, Rush has torn his ACL and will miss the entire season:
Brandon Rush is out for the season with torn ACL blog.sfgate.com/warriors/2012/…
— Rusty Simmons (@Rusty_SFChron) November 3, 2012
Rush may not be the Warriors' biggest name or highest scorer, but he plays a major role in the team's rotation. He's a three-point ace (career 41.3 percent from beyond the arc), and he's an above-average defender on the wing. On top of that, he's a modest role player who is simply happy to play his minutes every night.
With Rush out of the lineup, Golden State will likely give more minutes to rookies Harrison Barnes and Draymond Green and give veteran Richard Jefferson an expanded role. No one is going to replace Rush's defensive presence, but there are only so many choices.
According to Simmons' full report, the Warriors could apply for a disabled player exception. This would possibly give them the freedom to add a free agent, but options are thin there as well.
So far this season, Rush was averaging seven points per game. His ability to play shooting guard or small forward made the lineup more versatile, and he made the team more tough overall.
Golden State's biggest concern this year is injuries. Stephen Curry is injury-prone, Andrew Bogut is fragile and now Rush is on the shelf. Despite having enough talent to compete, one bad injury (possibly Rush's) could derail a hopeful season for Mark Jackson's team.