Well, this is something:
The chances that Nick Saban goes over this in detail for us are nil, so there's no good way to know what this is about. However according to Bruce Feldman, it's the just a precaution:
Re: AJ McCarron's walking boot. Am told by a source McCarron's fine. He practiced all week. #Bama— Bruce Feldman (@BFeldmanCBS) August 31, 2013
Despite Feldman's report, this will probably be an ongoing story line through the game against Virginia Tech. Knowing ESPN, every time he drops back, plants to throw, or runs out of the pocket, someone will be analyzing his feet and ankles on a high speed camera to find any possible evidence of trouble.
The Tide could just have McCarron hand off to the 15 or so five-star running backs the team has on every play and still cover the 20-point spread, so it doesn't matter a ton this weekend. We'll keep an eye on it, but if he looks like his normal self in Atlanta, you can probably just forget this ever happened.
A source told ESPN's Mark Schlabach that the boot is for an ingrown toenail. An Alabama spokesman told ESPN that it's merely precautionary.