Joe Staley is the starting left tackle for the 49ers and has had a pretty solid season defending the blind side for Alex Smith. But Staley went down early on their first snap of the game for the Niners, and he had to head to the locker room. It was later confirmed that Staley had suffered a head injury and will be out for the rest of the game today.
Staley being out means that Alex Boone will take his place at left tackle. The 49ers probably don't need Staley today, but they could use him in future contests, so they'll have to hope the injury isn't serious.