Brown (ankle) has left Sunday's game against the Cardinals, Nick Wagoner of ESPN.com reports.
Brown's departure will leave the 49ers down both of their starting safeties going into halftime, as Talanoa Hufanga (wrist) has already been ruled out for the remainder of the game. Brown will try to return in the second half, however, if he is unable to, Malik Mustapha will likely handle snaps at free safety.