Pete is absolutely right - you probably won't have to deal with anything beyond blisters, scratches, and sunburn - routine problems that are easily solved by a few bandaids. Sprained ankles do happen, and unfortunately demobilize the victim. It is also true that you can't usually differentiate between a sprained and a broken ankle in the field, so basically you treat them all the same.

At this point, your planning will pay off. The question isn't just about your FAK - that neat little bundle of goodies you have assembled, but your entire response to an injury that keeps one or more of your party from walking out. Essentially everything at your disposal becomes potential items for treatment- Improvisation is all important.

What would you use for a splint? Usually very fine splints can be improvised (We once did a great cervical collar with a piece of blue foam pad and duct tape). Aluminum pack stays can be pressed into service - on some of my packs I have actually drilled holes at the ends to facilitate their use for this. You need to look over your entire kit and think how you would improvise various items.

Most discussions of injuries always admonish you to treat for shock. This is fairly easy in an urban setting, but can be tricky out in the woods. There will be a delay of at least a few hours, if not a day or so, before your casualty can be transported. Can you keep this person stable for that period? You may need to basically set up camp right there. Can you do that? How will you feed everyone if that is necessary?

Basically you need to plan before hand as to how you will deal with a major injury. Different groups in different kinds of terrain will adopt widely varying strategies, but you should consider everything from immediate patient survey and treatment, to notification and requests for outside assistance, to eventual recovery of the victim. Prior planning prevents poor performance.