Tip to jedye for posting a FanShot about this. It is very hard to defend Bob against the people who claim he is "injury prone." Other than the 2006 season, Bob has played about as regularly as any other safety. However, when stuff like this pops up, it just reinforces the perception that he is brittle. Bob is the reigning Defensive Player of the Year. This means the NFL is now without last year's MVP (Tom Brady) and the reigning DPoY.
The plus side with Bob is he will be back, but high ankle sprains take time to heal. And knowing the Colts recent trend of dealing with player injuries, Bob will likely not be back and 100% ready until maybe the home game against Houston, which is in mid-November.
Melvin Bullitt or Matt Giordano will step in and play strong safety while Bob recovers.