It is not known what the illness is or how long he has been battling it, but based on the coaches comments below, it's possible he's been dealing with it for a while and that although he's played through it, it's become increasingly difficult to do so.
Media and fans have put an awful lot of pressure on athletes to preform through everything that others in humanity tend to get a pass on and despite the fact the Keefe was secretive regarding the circumstances of Matthews ailments it's nobodies business what he is going through and people should just wish him better health.
Is the timing suspect? Sure. As a fan should he be cut some slack? To an extent, yes. A small percentage of people are lucky to play professional sports and an even smaller percentage plays professional hockey and in the National Hockey League. In the time that Matthews has been a member of the Maple Leafs he's never shied away from excuses or explanations for his play. As a fanbase, even if they're at their wits end, having him healthy should be the first thing in the front of everyone's mind, especially in the long term.