Blame freak blow to head for Kolb's exit

Kevin Kolb's durability was a question mark when the Arizona Cardinals acquired him during the offseason. It's still a question mark after foot and toe injuries sidelined Kolb for four games.

The injury Kolb suffered Sunday had less to do with durability than bad luck.

The San Francisco 49ers' Justin Smith beat Cardinals guard Daryn Colledge with a power move to sack Kolb and force a fumble. As Kolb fell to the ground, his head dropped into Ahmad Brooks' path. Brooks' lower leg struck Kolb in the head as Brooks ran after the football.

John Skelton is playing quarterback for the Cardinals. Kolb has returned to the sideline after leaving for tests. It's unclear whether Kolb will return.

The 49ers, meanwhile, have lost left tackle Joe Staley. Alex Boone is playing left tackle. I'll update that injury once more is known.