Pure speculation on my part but given his age, how long he was out, and no mention of any type of surgery I would guess he had some kind of stress reaction or even a stress fracture in his lower leg. The recovery time is usually 2-3 months and pretty much just involves staying off of the affected leg so the bone can heal. The club released a video of Jack maintaining fitness while running on one of those weight reducing treadmills and they had him strapped to a "exogen bone healing system" at one point in the video. I couldn't find any official word so just my best guess.