Admittedly, I'm not always comfortable discussing injuries at this time of year. The William Nylander situation is unique, even by Toronto standards:
According to multiple sources, William Nylander's absence from the Toronto lineup has to do with a migraine so severe that team doctors tested to see if he suffered a concussion. One of the reasons for the secrecy around his situation is that an actual diagnosis has been hard to come by, whether it is a migraine, a concussion or something else that could affect that area.